Use gpgrt_b64dec in libgpg-error.

* src/b64dec.c: Remove.
* src/Makefile.am (main_sources): Remove b64dec.c.
* src/data-identify.c (pgp_binary_detection): Use gpgrt_b64dec in
libgpg-error.

gpgrt_b64dec is available in libgpg-error 1.27 and it is already in
use by JSON support.

• GnuPG-bug-id: T7180
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>