ldb: Remove the ability for Samba to compile against a system LDB

ldb: Remove the ability for Samba to compile against a system LDB

Samba will either provide the LDB to the system, or use a
private ldb, we will not use any other LDB from the system.

This is essentially equilvilant to the patch Debian has used
for Samba 4.17 and later, named "Force-LDB-as-standalone.patch"

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>