go/parser: Remove unused Parse* functions. Simplified ParseExpr signature.

Only ParseFile, ParseDir, and ParseExpr are used in the tree.
If partial parsing of code is required, it is fairly simple
to wrap the relevant piece of code into a dummy package for
parsing (see parser.ParseExpr).

Also: minor cleanups.

R=rsc
CC=golang-dev
https://golang.org/cl/5535055