Revert "go/ast, go/parser: parse alias declarations"

Revert "go/ast, go/parser: parse alias declarations"

This reverts commit 57ae83307fc4cb90338b39dcc6fe3c61ee8ce0b7.

Reason: Decision to back out current alias implementation.

For #16339.

Change-Id: I7bcc04ac87ea3590999e58ff65a7f2e1e6c6bc77
Reviewed-on: https://go-review.googlesource.com/32823
Reviewed-by: Matthew Dempsky <mdempsky@google.com>