SNMP: revert serial_number addition to _cisco-metadata mixin

Adding serial_number to _cisco-metadata.yaml turned out to suppress the
leaf-level _cisco-catalyst.yaml symbols list during profile merging, so
cisco-catalyst-profiled devices stopped extracting chassisSerialNumberString
(the previously-working source). The metric mixin scope was already
narrow benefit: the only known cisco_switch_1300 / c9407r devices now
match cisco-catalyst directly, and the other inheritors of _cisco-metadata
(cisco-firepower, cisco-firepower-asa, cisco-ironport-email) don't expose
entPhysicalSerialNum in their test fixtures.

Keep the entPhysicalSerialNum fallbacks in _cisco-catalyst.yaml and
cisco-sb.yaml only.

Author: ian28223

snmp/changelog.d/23498.fixed

@@ -1 +1 @@
-Add ``entPhysicalSerialNum`` fallbacks for Cisco ``serial_number`` metadata. The Cisco Catalyst and Cisco Small Business (cisco-sb) profiles now resolve ``serial_number`` from ``entPhysicalSerialNum`` at entPhysical indexes ``67109120`` (Catalyst 1300 / CBS350) and ``1``; the generic Cisco profiles inherit ``entPhysicalSerialNum.1`` via the shared metadata mixin. Adds new Catalyst 9200 (``1.3.6.1.4.1.9.1.3079``) and Catalyst 1300 (``1.3.6.1.4.1.9.1.3233``, ``1.3.6.1.4.1.9.1.3236``) sysobjectids to the cisco-catalyst profile.
+Cisco Catalyst and Cisco Small Business (cisco-sb): use ``entPhysicalSerialNum`` at entPhysical indexes ``67109120`` (Catalyst 1300 / CBS350) and ``1`` as ``serial_number`` fallbacks when the primary chassis serial source is absent. Adds new Catalyst 9200 (``1.3.6.1.4.1.9.1.3079``) and Catalyst 1300 (``1.3.6.1.4.1.9.1.3233``, ``1.3.6.1.4.1.9.1.3236``) sysobjectids to the cisco-catalyst profile so those devices match and report a serial.

snmp/datadog_checks/snmp/data/default_profiles/_cisco-metadata.yaml

@@ -5,14 +5,6 @@ metadata:
     fields:
       vendor:
         value: "cisco"
-      serial_number:
-        symbol:
-          # ENTITY-MIB: entPhysicalSerialNum at entPhysicalIndex 1 — common chassis index.
-          # Source: https://circitor.fr/Mibs/Html/ENTITY-MIB.php
-          # Catalyst 1300 / CBS-derived chassis (which use entPhysicalIndex 67109120) match the
-          # cisco-catalyst profile, which provides its own serial_number lookup at that index.
-          OID: 1.3.6.1.2.1.47.1.1.1.1.11.1
-          name: entPhysicalSerialNum
       version:
         symbol:
           OID: 1.3.6.1.2.1.1.1.0