Move definition of standard collations from initdb to pg_collation.dat

While working on [0], I was wondering why the collations ucs_basic and
unicode are not in pg_collation.dat. I traced this back through
history, and I think this was just lost in a game of telephone.

The initial commit for pg_collation.h (414c5a2ea6) has only the default
collation in pg_collation.h (pre .dat), with initdb handling everything
else. Over time, additional collations "C" and "POSIX" were moved to
pg_collation.h, and other logic was moved from initdb to
pg_import_system_collations(). But ucs_basic was untouched. Commit
0b13b2a771 rearranged the relative order of operations in initdb and
added the current comment "We don't want to pin these", but looking at
the email[1], I think this was more a guess about the previous intent.

I suggest we fix this now; see attached patch.

[0]:
https://www.postgresql.org/message-id/flat/1293e382-2093-a2bf-a397-c04e8f83d3c2%40enterprisedb.com

[1]: https://www.postgresql.org/message-id/28195.1498172402%40sss.pgh.pa.us