[analyzer] Remove deprecated option VirtualCall:PureOnly (#131823)

VirtualCallChecker.cpp implements two related checkers:
- `optin.cplusplus.VirtualCall` which reports situations when
constructors or destructors call virtual methods (which is bugprone
because it does not trigger virtual dispatch, but can be legitmate).
- `cplusplus.PureVirtualCall` reports situations when constructors or
destructors call _pure_ virtual methods, which is an error.

Six years ago these two bug types were both reported by the same checker
(called `optin.cplusplus.VirtualCall`) and it had an option called
`PureOnly` which limited its output to the pure case.

When (in 2019) the two checker parts were separated by the commit
d3971fe97b64785c079d64bf4c8c3e2b5e1f85a1, the option `PureOnly` was
preserved for the sake of compatibility, but it is no longer useful
(when it is set to true, it just suppresses all reports from
`optin.cplusplus.VirtualCall`) so it was marked as deprecated.

I'm removing this deprecated option now because it is no longer relevant
and its presence caused minor complications when I was porting
`VirtualCallChecker.cpp` to the new multipart checker framework
(introduced in 27099982da2f5a6c2d282d6b385e79d080669546).

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