Remove _generate_assert_error_msg and its usage now we have pytest assert rewriting

[coretl]

After #714, this function is no longer necessary:

ophyd-async/src/ophyd_async/testing/_assert.py

Lines 10 to 18 in c781e27

	def _generate_assert_error_msg(name: str, expected_result, actual_result) -> str:
	WARNING = "\033[93m"
	FAIL = "\033[91m"
	ENDC = "\033[0m"
	return (
	f"Expected {WARNING}{name}{ENDC} to produce"
	+ f"\n{FAIL}{expected_result}{ENDC}"
	+ f"\nbut actually got \n{FAIL}{actual_result}{ENDC}"
	)

Remove it, and change all assert x, _generate_assert_error_msg(...) to bare assert x