build: Drop redundant AC_SUBST macros

Variables that are declared with AC_ARG_VAR macro are substituted via
AC_SUBST macro.
PKG_CHECK_MODULES macro already has AC_ARG_VAR(${PACKAGE}_CFLAGS) and
AC_ARG_VAR(${PACKAGE}_LIBS).

build-aux/m4/bitcoin_find_bdb48.m4

@@ -91,7 +91,6 @@ AC_DEFUN([BITCOIN_FIND_BDB48],[
     fi
   fi
   if test "$use_bdb" != "no"; then
-    AC_SUBST(BDB_LIBS)
     AC_DEFINE([USE_BDB], [1], [Define if BDB support should be compiled in])
     use_bdb=yes
   fi