Fix up confusion over how to use EXTRA_INSTALL.

Fix up confusion over how to use EXTRA_INSTALL.

Some makefiles were trying to do this:

temp-install: EXTRA_INSTALL=contrib/test_decoding

but that no longer works as of commit aa019da52: the macro is now
consulted by the checkprep target, one level down, and apparently
gmake doesn't propagate such macro settings recursively.

The problem is masked since 42e61c774 because pgxs.mk also sets up
EXTRA_INSTALL, and correctly applies it to the checkprep target.

Unfortunately I'd not risked back-patching that to before v11.
Since aa019da52 was pushed back to v10, it broke test_decoding
there (the only module for which this actually makes a difference
at present).

Hence, back-patch 42e61c774 to v10.  Also, remove some demonstrably
useless settings of EXTRA_INSTALL in v10 and v11 (they'd already
been cleaned up in HEAD).

Per buildfarm.

Discussion: https://postgr.es/m/CAEepm=1pEJdwv6DSGmOfpX0EaX7L7sT28c1nXpqvQvmLfEWb1g@mail.gmail.com