[#E03B996AE9A11D2AAD8BA52E0B752DF99E8D4AAA]
Remove the value on an element
iterator unset(iterator pos) noexcept;
This function removes the value of an element at the specified position. After the call returns, `has_value` for the element is false.
All iterators that are equal to `pos` or come after are invalidated.
url u( "?first=John&last=Doe" );
u.encoded_params().unset( u.encoded_params().begin() );
assert( u.encoded_query() == "first&last=Doe" );
Linear in `this->url().encoded_query().size()`.
Throws nothing.