Prevent calling new ftl::Optional

This class inherits from std::optional, which does not have a virtual
destructor. As such, deleting an object of ftl::Optional using a pointer
to its base class has undefined behavior. Prevent this by removing new
from ftl::Optional. This is generally not the right way to use it
anyway, and in fact this builds as is.

Delete new[] while we're at it. This should be enough of a signal not to
try to heap allocate these. It's still possible for a programmer to
circumvent these deletions, but they prevent the straightforward
(broken) use case.

Bug: 261035092
Test: make
Change-Id: Iafdaf98ed88920162af2b445caf0e4a69be51ab5