fix(build): clean up redundant warning sources

[darion-yaphet]

• Remove redundant pthread linkage from protoc-gen-mcpack targets and quiet compiler warnings from unused rpcz/couchbase code paths and non-x86 iobuf builds.

What problem does this PR solve?

Issue Number: resolve

Problem Summary:
This PR cleans up redundant build configuration and compiler warning sources. protoc-gen-mcpack already links through the configured dynamic/static brpc target and
dynamic libraries, so the explicit pthread linkage is unnecessary. A few warning-only code paths also produced noise from unused rpcz/couchbase symbols and the non-
x86 iobuf fallback warning.

What is changed and the side effects?

Changed:

• Removed explicit pthread linkage from protoc-gen-mcpack CMake targets.
• Removed an unused span_type_str block from rpcz annotation printing.
• Marked DEFAULT_VBUCKET_COUNT as [[maybe_unused]].
• Removed the non-x86_64 #warning from iobuf fallback code.
• Ensured src/brpc/couchbase.cpp ends with a newline.

Side effects:

• Performance effects: None expected.
• Breaking backward compatibility: None expected.

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
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