Remove process object from PAL

[huoyaoyuan]

The pseudo handle for current process is still used by various api. There's no need to keep the object and ProcLocalData behind.

[Copilot]

Pull request overview

This PR removes the PAL “process object” (and its per-process local data) and drops session-ID tracking, simplifying PAL process-handle semantics to effectively only support the pseudo-handle for the current process.

Changes:

• Removed the PAL otiProcess object type / CProcProcessLocalData and the initial-process IPalObject backing data.
• Removed gSID / GetCurrentSessionId and stopped calling getsid() during PAL init.
• Updated TerminateProcess / PROCEndProcess and DuplicateHandle paths to treat the current-process pseudo-handle as the only supported “process handle” on Unix PAL.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/coreclr/pal/src/thread/process.cpp	Removes process object backing data; changes process termination path to be current-process-only; removes session-id API.
src/coreclr/pal/src/init/pal.cpp	Stops initializing gSID (removes getsid(gPID)).
src/coreclr/pal/src/include/pal/procobj.hpp	Removes process-object declarations and CProcProcessLocalData type.
src/coreclr/pal/src/include/pal/process.h	Removes gSID and PROCGetProcessIDFromHandle declaration; updates comments.
src/coreclr/pal/src/include/pal/palinternal.h	Removes GetCurrentSessionId declaration.
src/coreclr/pal/src/include/pal/corunix.hpp	Removes otiProcess from PAL object type enum.
src/coreclr/pal/src/handlemgr/handleapi.cpp	Removes process-handle validation via process object; changes duplication behavior for pseudo process handle.