Revert "Makefile change for cross compiling. (raysan5#3176)"

This reverts commit 70286c7.

Turns out this is not needed!
Can be done by running make like so:
```
make PLATFORM=PLATFORM_DESKTOP OS=Windows_NT CC=/llvm/bin/aarch64-w64-mingw32-gcc AR=/llvm/bin/aarch64-w64-mingw32-ar
```

src/Makefile

@@ -308,17 +308,6 @@ ifeq ($(PLATFORM),PLATFORM_ANDROID)
     AR = $(ANDROID_TOOLCHAIN)/bin/llvm-ar
 endif
 
-# This section is here to help handle cross compilation. eg build on Linux for Windows. If you are doing this, you better know what you are doing!
-ifdef TARGET_OS
-    ifdef CROSS_CC
-        CC = ${CROSS_CC}
-    endif
-    ifdef CROSS_AR
-        AR = ${CROSS_AR}
-    endif
-    PLATFORM_OS = ${TARGET_OS}
-endif
-
 # Define compiler flags: CFLAGS
 #------------------------------------------------------------------------------------------------
 #  -O1                      defines optimization level