[backport#17154 2/3][wallet] Remove return value from CommitTransaction()

Summary:
CommitTransaction returns a bool to indicate success, but since commit
b3a74100b8 it only returns true, even if the transaction was not
successfully broadcast. This commit changes CommitTransaction() to return
void.

All dead code in if (!CommitTransaction()) branches has been removed.

https://github.com/bitcoin/bitcoin/pull/17154/commits/d1734f9a3b138ab046f38ee44a09bc3847bf938a

Depends on D7112

Partial backport of Core PR17154

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7113