Revert "CI: Manage multibyte characters in syntax tests"

This reverts commit 0fde6aebdd.

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>

src/testdir/util/screendump.vim

@@ -105,7 +105,6 @@ enddef
 "	    some dictionary with some state entries;
 "	    the file contents of the newly generated screen dump;
 "	    the zero-based number of the line whose copies are not equal.
-" (See an example in runtime/syntax/testdir/runtest.vim.)
 "
 " The file name used is "dumps/{filename}.dump".
 "