Do not copy gcc dlls from dist-extras in windows binary-dist

Use versions from the build system rather than win-extras (which downloads
from opensuse)

Remove gcc dlls from downloaded nightly on appveyor

(cherry picked from commit c1e565c)
ref #15285

Author: tkelman

Makefile

@@ -438,7 +438,7 @@ endif
 
 ifeq ($(OS), WINNT)
 	[ ! -d dist-extras ] || ( cd dist-extras && \
-		cp 7z.exe 7z.dll libexpat-1.dll zlib1.dll libgfortran-3.dll libquadmath-0.dll libstdc++-6.dll libgcc_s_s*-1.dll libssp-0.dll $(bindir) && \
+		cp 7z.exe 7z.dll libexpat-1.dll zlib1.dll $(bindir) && \
 	    mkdir $(DESTDIR)$(prefix)/Git && \
 	    7z x PortableGit.7z -o"$(DESTDIR)$(prefix)/Git" && \
 	    echo "[core] eol = lf" >> "$(GITCONFIG)" && \

contrib/windows/msys_build.sh

@@ -105,6 +105,12 @@ rm -f usr/bin/libjulia.dll
 rm -f usr/bin/libjulia-debug.dll
 
 if [ -z "$USEMSVC" ]; then
+  rm -f usr/bin/libgcc_s_s*-1.dll
+  rm -f usr/bin/libgfortran-3.dll
+  rm -f usr/bin/libquadmath-0.dll
+  rm -f usr/bin/libssp-0.dll
+  rm -f usr/bin/libstdc++-6.dll
+
   if [ -z "`which ${CROSS_COMPILE}gcc 2>/dev/null`" ]; then
     f=$ARCH-4.9.2-release-win32-$exc-rt_v4-rev3.7z
     checksum_download \