Remove undefined parallel code from figure_management

[e10harvey]

Summary of changes:

• Remove parallel branch from save_all_figures
• Remove timeout and raise_on_timeout parameters from save_all_figures
• Update unit tests accordingly
• Fix matplotlib version to <= 3.8.4

Fixes #88 Fixes #109

[e10harvey]

@bbean23, @braden6521: Sorry for the spam. I'll try using this PR to resolve #109 too.

[e10harvey]

@bbean23, @braden6521: This one is ready for another review.