Remove pg_dump/pg_dumpall support for dumping from pre-8.0 servers.

Remove pg_dump/pg_dumpall support for dumping from pre-8.0 servers.

The need for dumping from such ancient servers has decreased to about nil
in the field, so let's remove all the code that catered to it.  Aside
from removing a lot of boilerplate variant queries, this allows us to not
have to cope with servers that don't have (a) schemas or (b) pg_depend.
That means we can get rid of assorted squishy code around that.  There
may be some nonobvious additional simplifications possible, but this patch
already removes about 1500 lines of code.

I did not remove the ability for pg_restore to read custom-format archives
generated by these old versions (and light testing says that that does
still work).  If you have an old server, you probably also have a pg_dump
that will work with it; but you have an old custom-format backup file,
that might be all you have.

It'd be possible at this point to remove fmtQualifiedId()'s version
argument, but I refrained since that would affect code outside pg_dump.

Discussion: <2661.1475849167@sss.pgh.pa.us>