Test: fuzzers: drop redundant fuzz test

pcmk_parse_interval_spec() is already (and properly) tested in strings_fuzzer.c

Previously, utils_fuzzer.c was present but unused. When the Pacemaker fuzzers
were recently moved to lib/*/fuzzers/* and OSS-Fuzz was updated to look there,
the file got picked up. This was mistakenly identified as a regression for some
reason.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=70823