Remove duplicate exports from Makefile

We already export these variables earlier in the Makefile, right
after they were 'declared'.  There is no point in doing so again.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Makefile

@@ -709,7 +709,7 @@ LIB_OBJS += $(COMPAT_OBJS)
 ALL_CFLAGS += $(BASIC_CFLAGS)
 ALL_LDFLAGS += $(BASIC_LDFLAGS)
 
-export prefix gitexecdir TAR INSTALL DESTDIR SHELL_PATH template_dir
+export TAR INSTALL DESTDIR SHELL_PATH
 
 
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