Remove dead feeest-file read code for old versions

[TheBlueMatt]

0.15.0 introduced a new feeest file format, and support for parsing
old versions was never fully added. We now simply fail to read the
old format, so remove the dead partial-implementation.

Follow up to #11273.

[murchandamus]

utACK 62e7c04

[jnewbery]

Why not remove nFileVersion from the TxConfirmStats::Read() function signature entirely?

[TheBlueMatt]

I figured it may be useful to have in a future change, so might as well leave it instead of creating more diff to change the function signature.

[jnewbery]

I'm able to hit the new check in #11273 by manually editing the nVersionRequired bytes in the fee_estimate.dat file:

...
2017-12-19 18:20:23 Read: incompatible old fee estimation data (non-fatal). Version: 140000

which means that TxConfirmStats::Read() doesn't get hit at all.

It'd be good to add a check in CBlockPolicyEstimator::Read() that nVersionThatWrote <= nVersionRequired, although that doesn't necessarily have to be done in this PR. That would ensure that we couldn't possibly enter TxConfirmStats::Read() if nVersionThatWrote < 149900.

it may be useful to have in a future change, so might as well leave it

My default preference is the other way. If code is unused it should be removed at the earliest opportunity.

Tested ACK 62e7c04

[meshcollider]

utACK 62e7c04

[laanwj]

utACK 62e7c04