go/types: simplify method set computation

After fixing #37081 we don't need to explicitly keep track of
field collisions in the method set computation anymore; we only
need to know which field (names) exists at each embedding level.
Simplify the code by removing the dedicated fieldSet data type
in favor of a simple string set.

Follow-up on https://golang.org/cl/218617; separate CL to make it
easier to identify a problem with these two changes, should there
be one.

Updates #37081.

Change-Id: I5c259c63c75a148a42d5c3e1e4860e1ffe5631bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/218618


Reviewed-by: Alan Donovan <adonovan@google.com>