[JIT] Remove buffer metadata serialization forward-compat gate (#49990)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49990
**Summary**
This commit removes the forward-compatibility gate for buffer metadata
serialization. It was introduced to allow versions of fbcode
binaries statically linked against older versions of PyTorch (without
buffer metadata in JIT) to deserialize archives produced by new versions
of PyTorch. Enough time has probably passed that these old binaries
don't exist anymore, so it should be safe to remove the gate.
**Test Plan**
Internal tests.
Test Plan: Imported from OSS
Reviewed By: xw285cornell
Differential Revision: D25743199
Pulled By: SplitInfinity
fbshipit-source-id: 58d82ab4362270b309956826e36c8bf9d620f081
