x/tools/gopls: respect the default checks of the staticcheck tool

[hyangah]

While testing for golang/vscode-go#1257, I noticed that the staticcheck offered from gopls includes ST1021 that's a non-default check in the standalone staticcheck tool. (I am using v0.6.6 gopls)

Since vscode-go is currently telling users to use staticcheck offered by gopls instead of using the lintOnSave feature when using staticcheck as the lint tool, I think it's desirable to respect the default offered by the standalone tool. Otherwise, we need to document the difference and why.

[gopherbot]

Change https://golang.org/cl/310449 mentions this issue: gopls/internal/hooks: respect the default checks of the staticcheck tool