Only deprecate some functions if BP Classic is not active

[imath]

12.0.0 is deprecating a lot of functions that Plugins might be using for a while. These deprecation notices are an interesting way to figure out the BP Classic plugin is needed.

Once the BP Classic plugin is activated, we then need to make sure these deprecation notices are not shown anymore, because:

• BP Classic include these functions
• BP Classic forces the Site to use pretty permalinks
• BP Classic forces the usage of the Legacy URL parser, so it's safe to add URL chunks behind functions building URLs.

This is achieved by putting deprecated functions behind a ! function_exists( 'bp_classic' ) check.

This task wasn't fully performed in #4954

Move some deprecated functions behind a specific check.
If ​BP Classic is active these functions won't need to throw deprecation notices.

Trac ticket: ​https://buddypress.trac.wordpress.org/ticket/8895

[imath]

In 13477:

Move some 12.0.0 deprecated functions behind a BP Classic plugin check

The BP Classic plugin will include these deprecated functions as the plugin is forcing the BP Legacy URL parser usage & pretty permalinks.

Fixes #8895