Simplify gettimeofday() fallback logic.

There's no known supported system needing 1 argument gettimeofday()
support. The test for it was added a long time ago (92c6bf9775). Remove.

Until now we tested whether a gettimeofday() fallback is needed when
targetting windows. Which lead to the odd result that HAVE_GETTIMEOFDAY only
being defined when targetting MinGW (which has gettimeofday() since at least
2007). As the fallback is specific to msvc, remove the configure code and
rename src/port/gettimeofday.c to src/port/win32gettimeofday.c.

While at it, also remove the definition of struct timezone, a forward
declaration of the struct is sufficient.

Reviewed-By: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/20220806000311.ywx65iuchvj4qn2k@awork3.anarazel.de

src/port/win32gettimeofday.c

@@ -1,8 +1,8 @@
 /*
- * gettimeofday.c
+ * win32gettimeofday.c
  *	  Win32 gettimeofday() replacement
  *
- * src/port/gettimeofday.c
+ * src/port/win32gettimeofday.c
  *
  * Copyright (c) 2003 SRA, Inc.
  * Copyright (c) 2003 SKC, Inc.