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

This reverts commit 05401b3f44.

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>

modules/hal_rpi_pico/CMakeLists.txt

@@ -4,7 +4,7 @@ include(ExternalProject)
 
 if(CONFIG_HAS_RPI_PICO)
   zephyr_library()
-  set_property(TARGET ${ZEPHYR_CURRENT_LIBRARY} PROPERTY C_STANDARD 11)
+  zephyr_library_compile_options(-std=gnu11)
 
   set(rp2_common_dir ${ZEPHYR_HAL_RPI_PICO_MODULE_DIR}/src/rp2_common)
   set(rp2xxx_dir ${ZEPHYR_HAL_RPI_PICO_MODULE_DIR}/src/${CONFIG_SOC_SERIES})