Drop Str2tagTest unit test that broke split / no-exceptions builds

The test referenced detail::can_compress_content_type, which lives below
the split BORDER in httplib.h and is therefore not visible to test.cc in
test_split / Windows-CMake builds. EXPECT_NO_THROW also expanded to a
try/catch that would not compile under -fno-exceptions. The OSS-Fuzz
reproducer in test/fuzzing/corpus already serves as the regression test
for #508087118 and is exercised by make fuzz_test.