Fix wallet selector size adjustment

Summary:

This PR sets QComboBox::AdjustToContents instead of default QComboBox::AdjustToContentsOnFirstShow for wallet selectors.

This is a backport of Core PR15167

Test Plan:
ninja && src/qt/bitcoin-qt

Create a new wallet with a very long name, and check that the name is not truncated in the dropdown selectors (in main window and in the console tab of the debug dialog)

Reviewers: #bitcoin_abc, Fabien, majcosta

Reviewed By: #bitcoin_abc, Fabien, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8356