[easy][PTE] Remove GetMutableSizePrefixed* functions
Summary: fb: Fix the error: https://www.internalfb.com/intern/sandcastle/job/9007199888273681/insights
Test Plan:
CI
```
~/fbsource/fbcode] eval $(fbpkg info --json smart.pytorch_mobile.backport_service.persistent | jq -r .build.config.build_command)
Downloaded 26538/30277 artifacts, 1.00 Gbytes, 4.9% cache miss (for updated rules)
Building: finished in 10:22.3 min (100%) 48817/48817 jobs, 48817/48817 updated
Total time: 11:13.5 min
More details at https://www.internalfb.com/intern/buck/build/f7743351-c166-4263-9140-bc59cbb39a37
BUILD SUCCEEDED
Reviewed By: qihqi, guangy10
Differential Revision: D35734987
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76019
Approved by: https://github.com/seemethere
