pgsql: Remove XLogFileInit() ability to unlink a pre-existing file.

Remove XLogFileInit() ability to unlink a pre-existing file.

Only initdb used it.  initdb refuses to operate on a non-empty directory
and generally does not cope with pre-existing files of other kinds.
Hence, use the opportunity to simplify.

Discussion: https://postgr.es/m/20210202151416.GB3304930@rfd.leadboat.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/421484f79c0b80209fa766eb00dbc2453a753273

Modified Files
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src/backend/access/transam/xlog.c     | 61 ++++++++++++++---------------------
src/backend/replication/walreceiver.c |  5 ++-
src/include/access/xlog.h             |  2 +-
3 files changed, 28 insertions(+), 40 deletions(-)