x/tools/gopls: deprecate the "tempModfile" setting

[findleyr]

This setting is not exposed in documentation, complicates the code, and can't be working correctly. Remove it.

I think it was intended to allow automatic go.mod mutation in combination with "allowModfileModification", but that is probably not working well, and significantly complicates our go command invocation.

In any case, I don't know how "tempModfile" is meant to be used, which is a sign that it should be removed...

[gopherbot]

Change https://go.dev/cl/518976 mentions this issue: gopls/internal/lsp/mod: remove TestModfileRemainsUnchanged

[gopherbot]

Change https://go.dev/cl/534919 mentions this issue: gopls: deprecate "tempModfile" and "expandWorkspaceToModule" settings