x/tools: fix staticcheck warnings

[stamblerre]

gopls enabled running diagnostics on an entire workspace, which surfaced > 200 staticcheck warnings in x/tools. We are disabling this behavior in gopls since it caused a real slowdown, but we should still fix all of the staticcheck warnings that we saw.

[gopherbot]

Change https://golang.org/cl/208098 mentions this issue: internal/lsp: don't run analyses on the entire view

[gopherbot]

Change https://golang.org/cl/208257 mentions this issue: all: fix some staticcheck errors