patch 9.2.0416: Unix: filename completion splits at space for single-file Ex commands

patch 9.2.0416: Unix: filename completion splits at space for single-file Ex commands

Problem:  SPACE_IN_FILENAME is defined on most platforms but not on Unix.
	  As a result, set_context_for_wildcard_arg() on Unix always resets the
	  completion pattern at white space for Ex commands that take a
          single file argument.
Solution: Drop the SPACE_IN_FILENAME ifdef (Maxim Kim)

fixes:  #18411
closes: #20090

Signed-off-by: Maxim Kim <habamax@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>