Remove TxBuilder allow_shrinking() and unneeded context param

[notmandatory]

Description

Remove wallet::TxBuilder::allow_shrinking() and unneeded TxBuilder context param.

Fixes #1374

Notes to the reviewers

The allow_shrinking function was the only one using the TxBuilder FeeBump context and it's useful to have CreateTx context functions available for building FeeBump transactions, see updated tests.

Changelog notice

Changed

• Removed TxBuilder::allow_shrinking() function.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[notmandatory]

@LLFourn I did a little more than just remove TxBuilder::allow_shrinking in this PR. If you think it's too much to remove the TxBuilder context I can put it back in, but then I'll need to either remove the two tests I updated or add the functions I needed to work for either context.

[ValuedMammal]

LGTM

Conceptually, I think allow_shrinking is still a valuable method to have in the api for TxBuilder, but for the moment it's worth reconsidering how this will work in relation to RBF, coin selection, etc.

[ValuedMammal]

ACK 1ab9204

[oleonardolima]

ACK 1ab9204

[oleonardolima]

ACK 97e2508

[ValuedMammal]

ACK 97e2508