Remove the unused GAP sync machinery from JuliaInterface

[fingolfin]

THREADSAFE_GAP_JL is never defined anywhere, so BEGIN_GAP_SYNC and END_GAP_SYNC always expanded to no-ops, and BeginGapSync and EndGapSync were compiled into the kernel extension without ever being declared or called. The only function that actually ran, InitGapSync, initialized a recursive mutex nobody locks and set a flag nobody reads. The file's own TODO: this is not actually fully implemented!! says as much.

Had the toggle ever been flipped on, the code would not have worked as intended anyway: in handle_jl_exception the END_GAP_SYNC() follows an ErrorMayQuit that never returns, and Func_JuliaGetGlobalVariableByModule can ErrorMayQuit in the middle of the guarded region, longjmp'ing past the unlock.

Removing it also gets rid of the local extern int jl_n_threads declaration, which conflicts with the _Atomic(int) declaration in julia.h since Julia 1.9. That went unnoticed as long as julia.h was not in scope in this file, but it breaks the build as soon as GAP's src/gasman.h includes julia.h, as proposed in gap-system/gap#6490 — that PR's CI with GAP.jl jobs currently fail with:

sync.c:24:16: error: conflicting type qualifiers for 'jl_n_threads'
julia.h:2172:34: note: previous declaration of 'jl_n_threads' with type 'int'

Verified locally that JuliaInterface builds cleanly both against a GAP with that PR applied and against GAP without it, so this remains compatible with released GAP versions.

AI disclosure: Claude Code (Opus 5) diagnosed the issue, drafted the change, and verified it locally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.30%. Comparing base (89ae110) to head (43ad7af).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1412      +/-   ##
==========================================
+ Coverage   83.28%   83.30%   +0.01%     
==========================================
  Files          63       62       -1     
  Lines        5073     5049      -24     
  Branches       30       28       -2     
==========================================
- Hits         4225     4206      -19     
+ Misses        848      843       -5     

Files with missing lines	Coverage Δ
pkg/JuliaInterface/src/JuliaInterface.c	95.00% <ø> (-0.24%)	⬇️
pkg/JuliaInterface/src/calls.c	83.72% <ø> (-0.49%)	⬇️
pkg/JuliaInterface/src/convert.c	92.59% <ø> (ø)

... and 1 file with indirect coverage changes

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