Revert "CI: Manage multibyte characters in syntax tests"

Revert "CI: Manage multibyte characters in syntax tests"

This reverts commit 0fde6aebddef5cb0428e85040994ba45e55cba99.

With "v9.1.2134" applied, arbitrary multibyte characters are
not replaced with spurious U+FFFD characters (typically when
the host system is under load); U+FFFD characters that are
intentionally written in an input file continue to remain
present for later output and comparison.  The workaround of
"v9.1.1592~3" is no longer necessary.

Also prefer "page_nr" to "nr" in syntax/testdir/runtest.vim

closes: #19348

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>