ref(web): remove unused form field classes from sentry.web.forms.fields (#108326)

## Summary
Remove dead code from `sentry.web.forms.fields`: `ReadOnlyTextWidget`,
`ReadOnlyTextField`, and `UserField`.

## Why these are safe to delete

- **ReadOnlyTextWidget** and **ReadOnlyTextField**: Added in commit
c5f224a (Aug 2014, "Implement managed users feature") for read-only
fields on managed users. Last used in `AccountSettingsForm` in
`web/forms/accounts.py` (managed users' username/email/name). That usage
was removed in commit **90d1d243fc4** (Jul 14, 2020, "chore(frontend):
Remove unused django forms" #19873). The admin `ChangeUserForm` usage
was removed in a031b32 (Jul 2019) when the user edit page was
converted to React. No references remain in Sentry or getsentry.

- **UserField** (Django form field): Last used in `web/forms/teams.py`
in `NewAccessGroupMemberForm` (`user = UserField()`). That file was
removed in commit **8d0505ac3b8** (Jun 3, 2015, "Remove usage of
AccessGroup"), so this field has been unused for ~10 years.

The `UserField` that remains in use is the DRF serializer in
`sentry.api.fields.user` (e.g. release owner field); it is unchanged.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: oioki

src/sentry/web/forms/fields.py

@@ -1,11 +1,6 @@
-from django.forms import CharField, EmailField, Field, TypedChoiceField, ValidationError
-from django.forms.utils import flatatt
-from django.forms.widgets import TextInput, Widget
-from django.utils.html import format_html
-from django.utils.safestring import mark_safe
+from django.forms import EmailField, TypedChoiceField, ValidationError
 from django.utils.translation import gettext_lazy as _
 
-from sentry.users.models.user import User
 from sentry.utils.email.address import is_valid_email_address
 
 
@@ -28,47 +23,6 @@ def validate(self, value):
             )
 
 
-class UserField(CharField):
-    class widget(TextInput):
-        def render(self, name, value, attrs=None, renderer=None):
-            if not attrs:
-                attrs = {}
-            attrs.setdefault("placeholder", "username")
-            if isinstance(value, int):
-                value = User.objects.get(id=value).username
-            return super().render(name, value, attrs=attrs, renderer=renderer)
-
-    def clean(self, value):
-        value = super().clean(value)
-        if not value:
-            return None
-        try:
-            return User.objects.get(username=value, is_active=True)
-        except User.DoesNotExist:
-            raise ValidationError(_("Invalid username"))
-
-
-class ReadOnlyTextWidget(Widget):
-    def render(self, name, value, attrs=None, renderer=None):
-        final_attrs = self.build_attrs(attrs)
-        if not value:
-            value = mark_safe("<em>%s</em>" % _("Not set"))
-        return format_html("<div{0}>{1}</div>", flatatt(final_attrs), value)
-
-
-class ReadOnlyTextField(Field):
-    widget = ReadOnlyTextWidget
-
-    def __init__(self, *args, **kwargs):
-        kwargs.setdefault("required", False)
-        super().__init__(*args, **kwargs)
-
-    def bound_data(self, data, initial):
-        # Always return initial because the widget doesn't
-        # render an input field.
-        return initial
-
-
 def email_address_validator(value):
     if not is_valid_email_address(value):
         raise ValidationError(_("Enter a valid email address."), code="invalid")