go/types: remove temp. work-around for issue 26390

[griesemer]

Reminder issue.

Remove the code around flag fixFor26390 in decl.go in favor of separate type-checking of receiver base types and their associated methods.

[griesemer]

Related: #23203

[gopherbot]

Change https://golang.org/cl/128435 mentions this issue: go/types: if base type for an alias is known, use it when needed

[gopherbot]

Change https://golang.org/cl/139422 mentions this issue: go/types: determine hasPtrRecv property from source rather than type

[gopherbot]

Change https://golang.org/cl/139897 mentions this issue: go/types: don't type-check method signatures eagerly anymore

[gopherbot]

Change https://golang.org/cl/139721 mentions this issue: go/types: fix recvPtr helper (follow-up on https://golang.org/cl/139422)

[gopherbot]

Change https://golang.org/cl/139425 mentions this issue: go/types: prepare for delayed type-checking of methods to when they are used

[gopherbot]

Change https://golang.org/cl/139899 mentions this issue: go/types: remove work-around for issue #26390

[gopherbot]

Change https://golang.org/cl/139900 mentions this issue: go/types: remove work-around for issue #26390