arch.7: Remove mention of PDP11_ENDIAN

AFACT, this was never accurate.  When there was support (such as it was)
for PDP endian it seems to have been spelled PDP_ENDIAN.

Reviewed by:	imp, kib, emaste
Sponsored by:	Innovate UK
Differential Revision:	https://reviews.freebsd.org/D52817

share/man/man7/arch.7

@@ -24,7 +24,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd July 14, 2025
+.Dd October 1, 2025
 .Dt ARCH 7
 .Os
 .Sh NAME
@@ -345,9 +345,6 @@ Common type size and endianness macros:
 .It Dv __LP64__ Ta 64-bit (8-byte) long and pointer, 32-bit (4-byte) int
 .It Dv __ILP32__ Ta 32-bit (4-byte) int, long and pointer
 .It Dv BYTE_ORDER Ta Either Dv BIG_ENDIAN or Dv LITTLE_ENDIAN .
-.Dv PDP11_ENDIAN
-is not used on
-.Fx .
 .El
 .Pp
 Architecture-specific macros: