vulkan : update ggml_vk_instance_validation_ext_available (llama/15666)

vulkan : update ggml_vk_instance_validation_ext_available (llama/15666)

* vulkan : update ggml_vk_instance_validation_ext_available

This commit updates ggml_vk_instance_validation_ext_available() to
check for VK_EXT_validation_features instead of
VK_KHR_portability_enumeration.

Based on how the returned boolean is used later in the code (to enable
both the validation layer and the VK_EXT_validation_features extension),
it appears the function may have been intended to check for the
validation layer features extension.

* remove try/catch

This was a left over from a previous iteration where I was explicitly
quering for a specific validation layer first, which would throw.

* update warning message about validation layers