doc: Remove bitness from bitcoin-qt help message and manpage

Summary:

Remove the (64-bit) from the bitcoin-qt help message.

Since removing the Windows 32-bit builds, it is no longer information
that is often useful for troubleshooting. This never worked for other
architectures than x86, and the only 32-bit x86 build left is the Linux
one. Linux users tend to know what architecture they are using.

It also accidentally ends up in the bitcoin-qt manpage.

This is a backport of Core PR17503

Test Plan: ninja && ninja doc-manpage-bitcoin-qt

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8266