Revert "modules: hal_rpi_pico: set -std=gnu11 in a toolchain independent way."

This reverts commit 05401b3f.

Introducing c-std setting with CMake way in the commit,
but the actual command line is below.

```
arm-zephyr-eabi-gcc -DKERNEL ... -std=gnu11 ...  -std=c99 ...
```

The setting `CONFIG_STD_C99` in Kconfig appends the `-std=c99`,
and (At least in gcc,) options are processed last-come-first,
so this setting was meaningless.

This will cause a build error, so we will revert it.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>