run-tests: Remove an ancient chceck for UCRT

It's enough to check for the _UCRT define these days, as long
as we are including a header that pulls in the base mingw headers.

Since 8b2ece59fccf0a65257b1ba2fe8d67d7e8a2e8c1 in mingw-w64 (May 6th
2019), the mingw-w64 headers define _UCRT. Since
94a28397eec79e54ce5a8354512f43da88f815d2 (committed on the same date),
the mingw-w64 headers use 0xE00 instead of 0x1400 for UCRT.

run-tests.sh

@@ -47,7 +47,7 @@ cd test
 HAVE_UWP=1
 cat<<EOF > is-ucrt.c
 #include <corecrt.h>
-#if __MSVCRT_VERSION__ < 0x1400 && !defined(_UCRT)
+#if !defined(_UCRT)
 #error not ucrt
 #endif
 EOF