Remove emode argument from XLogFileRead() and XLogFileReadAnyTLI()

Remove emode argument from XLogFileRead() and XLogFileReadAnyTLI()

This change makes the code slightly easier to reason about, because
there is actually no need to know if a specific caller of one of these
routines should fail hard on a PANIC, or just let it go through with a
DEBUG2.

The only caller of XLogFileReadAnyTLI() used DEBUG2, and XLogFileRead()
has never used its emode.  This can be simplified since 1bb2558046cc
that has introduced XLogFileReadAnyTLI(), splitting both.

Author: Yugo Nagata
Discussion: https://postgr.es/m/20240906201043.a640f3b44e755d4db2b6943e@sraoss.co.jp