pg_resetwal: Use new routine to retrieve data of PG_VERSION

pg_resetwal: Use new routine to retrieve data of PG_VERSION

pg_resetwal'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 pg_resetwal is that the first line of PG_VERSION
is read and reported in the error report generated when the major
version read does not match with the version pg_resetwal has been
compiled with.  The new logic preserves this property, without changes
to neither the error message nor the data used in the error report.

Note that as a chdir() is done within the data folder before checking the
data of PG_VERSION, get_pg_version() needs to be tweaked to look for
PG_VERSION in the current folder.

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