tests: posix: xsi_single_process: remove unnecessary min_ram filter

The xsi single process test suite requires something like 256 bytes
of heap space. There is no legitimate reason to require 24kB of
ram.

This gets rid of a twister error

```
Error found: portability.xsi.single_process.newlib on
qemu_cortex_m0/nrf51822 (Not enough RAM but is one of the integration
platforms)
```

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>