Fix multiple definition linker error in unit tests

In the same spirit as ee7eb0e, this removes the extra
definition in postgres_test.c.

This is uncovered by building with GCC 10, where -fno-common is the new
default [1][2] (vis a vis -fcommon). I could also reproduce this by
turning on "-fno-common" in older releases of GCC and Clang.

Backpatch to 6X_STABLE.

(cherry picked from commit 108504f)

src/backend/tcop/test/postgres_test.c

@@ -40,7 +40,6 @@ errfinish_impl(int dummy __attribute__((unused)),...)
 		will_return(errstart, true);\
     } \
 
-const char *progname = "postgres";
 
 /* List with multiple elements, return FALSE. */
 static void