Flush a stray definition of $(DLLTOOL).

Even if this is needed, it'd be configure's responsibility to set it.

src/makefiles/Makefile.cygwin

-DLLTOOL= dlltool
 # src/makefiles/Makefile.cygwin
+
 ifdef PGXS
 BE_DLLLIBS= -L$(libdir) -lpostgres
 else
@@ -43,4 +43,4 @@ endif
 
 # Rule for building a shared library from a single .o file
 %.dll: %.o
-	 $(CC) $(CFLAGS)  -shared -o $@ $<  $(LDFLAGS) $(LDFLAGS_SL) $(BE_DLLLIBS)
+	$(CC) $(CFLAGS)  -shared -o $@ $<  $(LDFLAGS) $(LDFLAGS_SL) $(BE_DLLLIBS)