refactor: Remove UB in prevector reverse iterators

rend() creates a pointer with offset -1. This is UB, according to the
C++ standard: https://eel.is/c++draft/expr.add#4:

    When an expression J that has integral type is added to [...] an
    expression P of pointer type, the result has the type of P.

    ... if P points to a (possibly-hypothetical) array element i of an
    array object x with n elements [...] the expressions P + J and J + P
    (where J has the value j) point to the (possibly-hypothetical) array
    element i+j of x if 0≤i+j≤n [...]

    Otherwise, the behavior is undefined.

Also, it is unclear why the functions exist at all, when stdlib utils
such as std::reverse_iterator{it} or std::views::reverse can be used out
of the box.

So remove them, along with the ubsan suppressions, that are no longer
used.

test/sanitizer_suppressions/ubsan

@@ -54,7 +54,6 @@ unsigned-integer-overflow:DecompressAmount
 unsigned-integer-overflow:crypto/
 unsigned-integer-overflow:MurmurHash3
 unsigned-integer-overflow:TxConfirmStats::EstimateMedianVal
-unsigned-integer-overflow:prevector.h
 unsigned-integer-overflow:InsecureRandomContext::rand64
 unsigned-integer-overflow:InsecureRandomContext::SplitMix64
 unsigned-integer-overflow:bitset_detail::PopCount
@@ -62,7 +61,6 @@ implicit-integer-sign-change:SetStdinEcho
 implicit-integer-sign-change:compressor.h
 implicit-integer-sign-change:crypto/
 implicit-integer-sign-change:TxConfirmStats::removeTx
-implicit-integer-sign-change:prevector.h
 implicit-integer-sign-change:verify_flags
 implicit-integer-sign-change:EvalScript
 implicit-integer-sign-change:serialize.h