Remove mingwcompat.c

We believe that the issues that this was working around have been
fixed in MinGW more than 5 years ago, so this isn't necessary anymore.

Discussion: https://www.postgresql.org/message-id/flat/20190719050830.GK1859%40paquier.xyz

Author: petere

src/backend/port/win32/Makefile

@@ -12,7 +12,7 @@ subdir = src/backend/port/win32
 top_builddir = ../../../..
 include $(top_builddir)/src/Makefile.global
 
-OBJS = timer.o socket.o signal.o mingwcompat.o
+OBJS = timer.o socket.o signal.o
 ifeq ($(have_win32_dbghelp), yes)
 OBJS += crashdump.o
 endif