doc: Remove bitness from bitcoin-qt help message and manpage
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 [[https://github.com/bitcoin/bitcoin/pull/17503 | 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

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