Revert "spec: document the underlying type of comparable"

This reverts CL 380854.

Per the conluding discussions on #50791. A follow-up will
document `comparable` more thoroughly.

For #50791.

Change-Id: I15db9051784a012f713e28d725c3b8bbfeb40569
Reviewed-on: https://go-review.googlesource.com/c/go/+/381076
Trust: Robert Griesemer <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: griesemer

doc/go_spec.html

@@ -820,8 +820,7 @@ <h3 id="Underlying_types">Underlying types</h3>
 
 <p>
 Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code>
-is one of the predeclared boolean, numeric, or string types, the predeclared
-type <code>comparable</code>, or a type literal,
+is one of the predeclared boolean, numeric, or string types, or a type literal,
 the corresponding underlying type is <code>T</code> itself.
 Otherwise, <code>T</code>'s underlying type is the underlying type of the
 type to which <code>T</code> refers in its <a href="#Type_declarations">type