Remove Cargo.lock.msrv

[dhardy]

We don't need this any more since the resolver is now version-aware. This change was already made to other rand repos (e.g. rust-random/rand#1672).

(I don't think we need to mention this in the CHANGELOG, or particularly care whether this gets merged before or after the release.)

[mstoeckl]

Looks reasonable. (I see rand later dropped the cargo generate-lockfile --ignore-rust-version step in favor of a fixed Cargo.lock, but rand_distr doesn't currently require an exact rc version for a specific dependency...)

[dhardy]

Fixing Cargo.lock was a separate thing. We don't need that so long as any pre-release versions are pinned (e.g. =1.2.3-pre.0).