runtime: remove unused frag calculation in ASAN poisoning

[qinlonglong123]

The variable frag is computed but never used in the ASAN poisoning logic below.
The code calculates the fragment size between the actual allocation slot and the requested size, optionally adjusting for the malloc header, but the result is discarded.
Remove this dead code since asanpoison and asanunpoison only require size and asanRZ.

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Patch Set 1:

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Patch Set 2: Auto-Submit+1 Code-Review+2 Commit-Queue+1

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Patch Set 2:

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Patch Set 2: -Commit-Queue

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This CL has failed the run. Reason:

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• go on master (change 793580)

To reproduce, try gomote repro 8678100183499456289.

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