pg_combinebackup: Use new routine to retrieve data of PG_VERSION

pg_combinebackup: Use new routine to retrieve data of PG_VERSION

pg_combinebackup's custom logic to retrieve the version number of a data
folder's PG_VERSION can be replaced by the facility introduced in
cd0be131ba6f.  This removes some code.

One thing specific to this tool is that backend versions older than v10
are not supported.  The new code does the same checks as the previous
code.

Author: Michael Paquier <[email protected]>
Reviewed-by: Masahiko Sawada <[email protected]>
Discussion: https://postgr.es/m/[email protected]