[doc] remove non-signaling mentions of BIP125

Our RBF policy is different from the rules specified in BIP125. For
example, the BIP does not mention Rule 6, and our Rule 4 uses the
(configurable) incremental relay feerate (distinct from the
minimum relay feerate). Those interested in our policy should refer to
doc/policy/mempool-replacements.md instead. These rules may also
continue to diverge with package RBF and other RBF improvements. Keep
references to the BIP125 signaling wrt sequence numbers, since that is
still correct and widely used. It is helpful to refer to this as "BIP125
signaling" since it is unambiguous and succint, especially if we have
multiple ways to signal replaceability in the future.

The rule numbers in doc/policy/mempool-replacements.md correspond
largely to those of BIP 125, so we can still refer to them like "Rule 5."

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