Remove COMPAT_TABLE_VERSION.

Remove COMPAT_TABLE_VERSION.

The compatibility version concept was introduced eight years ago in
commit (62d228ec), and was never needed. In fact, COMPAT_TABLE_VERSION
has always been zero so far. This commit gets rid of it.

Given a library version and the on-disk table version, we can still
tell whether the table can be opened by comparing the MIN_TABLE_VERSION
and the CURRENT_TABLE_VERSION numbers of the library against the
value stored in the table (i.e., the CURRENT_TABLE_VERSION value of
the library that created the table).