llama : add llama_lora_adapter_clear

[ngxson]

Ref discussion: #8636 (comment)

llama_lora_adapter_clear() can be used when user want to switch the adapter, but don't know which adapters are loaded into llama_context to be removed.

For simple task switching application, user can write their own code based on this function:

void switch_my_adapter(struct llama_context * ctx, struct llama_lora_adapter * adapter) {
  llama_lora_adapter_clear(ctx);
  llama_lora_adapter_set(ctx, adapter, 1.0);
}

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[zhipenghan]

Thank you for implementing this change. Now, users can manage adaptors outside of the library as if the existing adaptors were invisible.

[ngxson]

Oops sorry I didn't modify the merge commit message to specify module name