refactor: standardize _in_memory_table_exists

[ncclementi]

I saw this comment https://github.com/ibis-project/ibis/pull/10067#pullrequestreview-2290642303 from @jcrist and I'm converting it into an issue so we don't loose track of it.

Where is suggested to change the default implementation to:

try:
    self.table(name)
    return True
except TableNotFound:
    return False

This https://github.com/ibis-project/ibis/pull/9695 was merged I'm not sure about the details of the implementation/refactor but this seems like it could be a nice consolidation of code