Teach plpgsql's "make clean" to remove generated test files.

Teach plpgsql's "make clean" to remove generated test files.

Copy the rules that src/test/regress/GNUmakefile uses for this purpose.
Since these files are .gitignore'd, the mistake wasn't obvious unless
you happened to look at "git status --ignored" in an allegedly clean
tree.

Oversight in commit 1858b105b.  No need for back-patch since that's
not in the back branches.