Simplify use of NDEBUG in etags.c

* lib-src/etags.c [!DEBUG]: Do not define NDEBUG,
as that’s conf_post.h’s job now.
[NDEBUG]: Do not redefine ‘assert’, as that works around
ancient bugs (e.g., sunos4 pcc) that don’t matter any more,
as can be seen that other Emacs source files that successfully
use assert.h without this workaround.
1 job for master in 53 minutes and 18 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #1634


Name Stage Failure
test-all Test
Makefile:315: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:290: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:963: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1