Fix log prefix hijack in c_cmis.py

[nate-nexthop]

Description

Fix logging in c_cmis.py to match neighboring files and to stop hijacking the syslog tag of any importing executables.

Motivation and Context

In python services using sonic_py_common.logger the syslog identifier already set up by the executable/service can be overwritten when importing other modules.

This log hijack is coming from a library: sonic_platform_base/sonic_xcvr/api/public/c_cmis.py

SYSLOG_IDENTIFIER = "CCmisApi"
...
helper_logger = logger.Logger(SYSLOG_IDENTIFIER)
# This is the only log that this file has:
    helper_logger.log_debug('key {} not present in VDM'.format(new_key))

Searching in the syslog of a switch running SONiC we see logs with the CCmisApi that shouldn't:

2025-11-11T13:20:03.440479+00:00 sonic CCmisApi: Failed to load platform Pcie module. Warning : No module named 'sonic_platform.pcie', fallback to load Pcie common utility.
2025 Nov 11 13:20:45.179454 gold228 NOTICE CCmisApi: System is ready

If any code imports c_cmis.py (indirectly for the most part) and has a logger session open already, their existing session will be re-opened with a new log tag, CCmisApi which is not desired.

To fix this we remove this logging code entirely because the function where it is used became dead in #556

Libraries (rather than executables) probably shouldn't be syslogging on their own, and they certainly shouldn't hijack the log tag of the including executable.

Resolves sonic-net/sonic-buildimage#24489

How Has This Been Tested?

Before the fix:
Open an interactive python session and run this code:

from sonic_py_common import logger
log = logger.Logger("NATE")
log.log_notice("Log 1")
import sonic_platform
log.log_notice("Log 2")

Watch the syslog output:

2025 Nov 11 13:47:29.002667 gold228-dut NOTICE NATE: Log 1
2025 Nov 11 13:48:28.406481 gold228-dut NOTICE CCmisApi: Log 2

Clearly the second log should also have had the tag NATE

After the fix:
Syslog entries should have been:

2025 Nov 11 13:47:29.002667 gold228-dut NOTICE NATE: Log 1
2025 Nov 11 13:48:28.406481 gold228-dut NOTICE NATE: Log 2

Additional Information (Optional)

[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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[nate-nexthop]

@prgeor and @mihirpat1 would you mind taking a look please?

[Copilot]

Pull Request Overview

This PR fixes a logging issue where the c_cmis.py library file hijacks the syslog identifier of any importing executable by removing unused logging code and dead functions.

Key Changes:

• Removed unused logger import and logging infrastructure that was overriding syslog identifiers
• Removed dead code (_update_dict_if_vdm_key_exists method) that became unused in a previous PR
• Removed unused BYTELENGTH constant

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