Remove short option -t for --theme

[shahzebsiddiqui]

The -t for --theme conflicts with sub commands specially buildtest it query --testpath. We want this option --testpath to have -t for --testpath but since -t is reserved for --theme lets not have a short option.

Let's remove the -t for --theme throughout the codebase and only use long option

See

buildtest/buildtest/cli/__init__.py

Line 308 in 260a35c

"-t",

We should do the following

• update bash completion script and remove -t option wherever its used for --theme
• Remove -t from documentation wherever its mentioned for --theme also check all .. command-output where its -t and replace with --theme
• Update any docstring or comment throughout the code base