var: drop the write-only variable index

[andypost]

Problem

The scheduled "Build (Fedora/Alpine · GCC/Clang)" workflow fails on both clang legs since the images moved to clang 23 (Alpine clang 23.1.1, Fedora clang 23.1.0); the gcc legs pass:

https://github.com/freeunitorg/freeunit/actions/runs/35585942071

src/nxt_var.c:75:29: error: variable 'nxt_vars' set but not used [-Werror,-Wunused-but-set-global]

The last green run (https://github.com/freeunitorg/freeunit/actions/runs/33384248955, 2026-08-31) used Alpine clang 22.1.8; the first red one (https://github.com/freeunitorg/freeunit/actions/runs/34830122484, 2026-09-14) was already on 23.x. No source change is involved. The clang 23 release notes add -Wunused-but-set-global as a new subgroup of -Wunused-but-set-variable (in -Wall) covering file-scope statics; configure adds -Werror, so the new warning is fatal.

Fix

The diagnostic is right, so delete the dead code rather than suppress the flag.

nxt_vars is a file-static pointer written once by nxt_var_index_init() and never read; git log -S nxt_vars -- src/nxt_var.c shows no reader in the whole history of the file. That makes nxt_var_index_init() dead at startup: it nxt_memalign(64, …)s an array of every registered variable, copies the hash into it, stores the pointer and returns. nxt_var_count exists only to size that array. Remove the index, the counter, the prototype in nxt_var.h and the call in nxt_runtime_create(); the surrounding goto fail chain is unchanged.

No user-visible change, so no CHANGES / changes.xml entry.

Verification

• Reproduced the error on unmodified origin/master in alpine:edge with clang 23.1.1 (./configure --cc=clang --openssl --tests && make unitd); with make -k it is the only error in the tree, so no other file trips the new warning.
• With this patch, the same container builds unitd cleanly; unitd --version reports 1.36.1 and a --no-daemon start brings up controller and router and exits cleanly on SIGTERM.
• Local no-regression builds: gcc 15.2.0 and clang 21.1.8 (make unitd), plus make tests under clang 21 with the C test binary passing.

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