[cleanup](load) remove bvar "g_memtable_input_block_allocated_size"

[kaijchen]

What problem does this PR solve?

Bvar "g_memtable_input_block_allocated_size" is no longer needed after #40912.
Calling MutableBlock::allocated_bytes() in Memtable::insert() has some performance penalties.
So we should remove it.

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[doris-robot]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[kaijchen]

run buildall

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[doris-robot]

TeamCity be ut coverage result:
Function Coverage: 38.02% (9898/26035)
Line Coverage: 29.21% (82816/283535)
Region Coverage: 28.33% (42514/150055)
Branch Coverage: 24.89% (21555/86584)
Coverage Report: http://coverage.selectdb-in.cc/coverage/61dc0f3864634f256f8f36771d021b229594f8d1_61dc0f3864634f256f8f36771d021b229594f8d1/report/index.html

[dataroaring]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.

[kaijchen]

run buildall

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[doris-robot]

TeamCity be ut coverage result:
Function Coverage: 38.30% (9976/26048)
Line Coverage: 29.43% (83513/283801)
Region Coverage: 28.57% (42960/150354)
Branch Coverage: 25.16% (21824/86742)
Coverage Report: http://coverage.selectdb-in.cc/coverage/fa648235f8d459bc550624f9a8224180618ce6c1_fa648235f8d459bc550624f9a8224180618ce6c1/report/index.html