cmd/vet: remove two unused parameters and simplify

cmd/vet: remove two unused parameters and simplify

The isStar and directory function parameters have been unused ever since
they were introduced. Remove them.

While at it, apply some other minor simplifications, such as simplifying
a HasPrefix if and using an early continue to unindent many lines of
code.

Change-Id: I8d57353e9ec10cdb59c5388cf6152ce0ec17bdba
Reviewed-on: https://go-review.googlesource.com/62030
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>