go/build: matchFile parameter returnImports is actually unused

[mvdan]

And has been ever since it was introduced in 2013 by @rsc. The godoc says:

// If returnImports is true and name denotes a Go program, matchFile reads
// until the end of the imports (and returns that data) even though it only
// considers text until the first non-comment.

Should the method actually use the bool parameter, or should it be removed from the signature and godoc? The func is currently called called with both true and false.

Found by https://github.com/mvdan/unparam.

[rsc]

You can delete the parameter. The code uses the file suffix instead, which is a better source of truth.

[gopherbot]

CL https://golang.org/cl/38265 mentions this issue.