Remove invalid UTF8 check from Repr::from_utf8_unchecked

[overlookmotel]

This PR removes the following code from Repr::from_utf8_unchecked:

compact_str/compact_str/src/repr/mod.rs

Lines 126 to 137 in d479863

	// There's an edge case where the final byte of this buffer == `HEAP_MASK`, which is
	// invalid UTF-8, but would result in us creating an inline variant, that identifies as
	// a heap variant. If a user ever tried to reference the data at all, we'd incorrectly
	// try and read data from an invalid memory address, causing undefined behavior.
	if bytes_len == MAX_SIZE {
	let last_byte = bytes[bytes_len - 1];
	// If we hit the edge case, reserve additional space to make the repr becomes heap
	// allocated, which prevents us from writing this last byte inline
	if last_byte >= 0b11000000 {
	repr.reserve(MAX_SIZE + 1)?;
	}
	}

In my opinion, this code should be removed for 2 reasons:

1. It is part of the safety contract for from_utf8_unchecked that buf is the bytes of a valid UTF8 string. This check penalizes users who follow the safety contract (which everyone should), in order to support users who don't (and therefore have opted in to UB).
2. This check alone is not enough to avoid UB anyway. If buf ends with the first byte of a multi-byte Unicode character, for example calling .as_str().chars().collect::<Vec<char>>() on the resulting CompactString will cause an out of bounds read.

[overlookmotel]

CI fails are I believe caused by tests/fuzz which are breaking the safety constraint of utf8_unchecked. If intention is to merge this, those tests should be removed/adapted.

I've fixed the proptest, but I don't know how to alter the fuzzer.