drivers/at: remove at_urc_isr module

[derMihai]

Description

The at_urc_isr module introduced with #14327 comes with a number of bugs. The issues are:

• improper use of fencing - accesses to at_dev_t::awaiting_response should use the C11 atomics, as there is little preventing an optimizing compiler from moving them around.
• The URC handling in ISR may catch command responses - as @aaltuntas points out, after an AT command is issued, the ISR handling is enabled and disabled multiple times, allowing it to wrongly intercept responses as URCs

As I am neither interested in this feature, nor able to produce enough test coverage, I am not willing (at least for the moment) to fix it. Therefore, until someone will fix this, I suggest It should either be removed, or marked as experimental and compilation should fail when enabled.

[derMihai]

I will close this, as #20423 solves this.