clean up cluttered context menu

[hyangah]

Context Menu looks cluttered and some of the comments are going to be obsolete
when the language server is used by default and it offers them with better code action/lenses/quick fixes.

Clean these up.

[hyangah]

This is non-critical but would be nice if done as we enable gopls by default.
Similarly we can disable some of the commands (e.g. go.fill.struct, etc) by using the when condition and setting the context (see example)

[gopherbot]

Change https://golang.org/cl/282675 mentions this issue: docs/settings.md: fix the settings for the context menu

[hyangah]

Looking at the command list, only go.fill.struct is the questionable one, and others aren't replaced by gopls yet.