Add new string ops fuzzers and remove old ones
CC @fcoUnda @learning-to-play
Adds new string ops fuzzers using the FuzzTest fuzzer-style, and removes the existing fuzzers for string split operations.

The current string split fuzzer are not hitting the logic of the string split operations, because the first argument they pass to the operation is set to be a string rather than a vector of strings. Consequently, the fuzzers are running into some blockers early in the code hindering the fuzzers in exploring the actual logic they target: https://storage.googleapis.com/oss-fuzz-coverage/tensorflow/reports/20230225/linux/proc/self/cwd/tensorflow/core/kernels/string_split_op.cc.html#L177