Remove unused variable

[rogerhu]

Fixes:

/Applications/Xcode.app/Contents/Developer/usr/bin/make -f objs/Makefile
cc -c -pipe  -O -Wall -Wextra -Wpointer-arith -Wconditional-uninitialized -Wno-unused-parameter -Wno-deprecated-declarations -Werror -g  -I../nginx-rtmp-module  -I src/core -I src/event -I src/event/modules -I src/event/quic -I src/os/unix -I /opt/homebrew/Cellar/pcre2/10.42/include -I ../openssl-1.1.1m/.openssl/include -I objs -I src/http -I src/http/modules -I ../nginx-rtmp-module \
		-o objs/addon/nginx-rtmp-module/ngx_rtmp_eval.o \
		../nginx-rtmp-module/ngx_rtmp_eval.c
../nginx-rtmp-module/ngx_rtmp_eval.c:87:25: error: variable 'k' set but not used [-Werror,-Wunused-but-set-variable]
    ngx_uint_t          k;