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 <[email protected]>

Author: cfriedt

tests/posix/xsi_single_process/testcase.yaml

@@ -28,7 +28,6 @@ tests:
       - CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE=256
   portability.xsi.single_process.newlib:
     filter: TOOLCHAIN_HAS_NEWLIB == 1
-    min_ram: 24
     extra_configs:
       - CONFIG_NEWLIB_LIBC=y
       - CONFIG_NEWLIB_LIBC_MIN_REQUIRED_HEAP_SIZE=256