[LibOS] Remove `migrated_envp`

[dimakuv]

Description of the changes

This variable is wrong and shouldn't be used.

But also see https://github.com/gramineproject/graphene/issues/2081 and the TODOs in this PR.

How to test this PR?

CI is enough. I modified sigprocmask_pending LibOS regression test, otherwise (if envp = {NULL}) it failed with:

sigprocmask_pending: error while loading shared libraries: libpthread.so.0: cannot open shared object file: No such file or directory

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This change is