[gui] do not enable pruning by default

Summary:
Enabling pruning by default on the intro screen, when the selected data directory does not have enough disk space available to store the entire blockchain, can cause accidental pruning. When the filesystem containing the data is not mounted when bitcoin-qt is started, the intro screen is shown with the usual data directory selected and the pruning option checked. If the file system is then mounted (e.g. external hard drive connected) and the user does not notice that pruning is selected, the blockchain is pruned after he clicks OK. See also https://github.com/bitcoin/bitcoin/issues/19291

Besides that, if a user selects pruning and then changes the data directory, it should not unselect the option.

This commit removes default pruning behavior introduced in D7956, no matter how much disk space is detected on the selected filesystem. Pruning can always be activated later if needed, from the menu.

Test Plan:
Run bitcoin-qt in a way that causes the intro screen to be displayed:
ninja && src/qt/bitcoin-qt -choosedatadir /does/not/exist

Select data directories with various available disk space (less than 10GB, between 10GB and the current blockchain size, more than the current blockchain size), and make sure that this does not automatically check the pruning option.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8187