Remove broken dumping support from portfolio build (#2470)

Remove broken dumping support from portfolio build (#2470)

Dumping support for portfolio builds was introduced in
84f26af22566f7c10dea45b399b944cb50b5e317 but as far as I can tell, the
implementation has already been problematic in the original implementation. The
current implementation has the following issues:

- Dumping with a portfolio build (even when using the single threaded
executable) results in a segfault. The reason is that the DumpChannel variable
is declared as an `extern` and exists for the duration of the program. The
problem is that it stores a `CommandSequence`, which indirectly may store
nodes. When shutting down CVC4, the destructor of `DumpC` is called, which
destroys the `CommandSequence`, which results in a segfault because the
`NodeManager` does not exist anymore. The node manager is (indirectly) owned
and destroyed by the `api::Solver` object.
- Additionally, adding commands to the `CommandSequence` is not thread safe (it
just calls `CommandSequence::addCommand()`, which in turn pushes back to a
vector [0] (which is not thread safe [1]). A single instance of `DumpChannel`
is shared among all threads (it is not declared `thread_local` [2]).
- The `CommandSequence` in `DumpC` was not used in the original implementation
and is still unused on master. The original commit mentions that the portfolio
build stores the commands in the `CommandSequence` but not why.

This commit removes the `CommandSequence` and related methods from `DumpC` to
avoid the issue when destroying `DumpChannel`. It disables dumping for
portfolio builds and adds a check at configure-time that not both options have
been enabled simultaneously.  Given that the option was not functional
previously, the problematic implementation, and the fact that the dump of
multiple threads wouldn't be very useful, disabling dumping for portfolio
builds is unlikely to be problem. An improvement that could be made is to
disable dumping support only for the pcvc4 binary and while enabling it for the
single-threaded version, even when using `--with-portfolio`. However, we
currently do not have the infrastructure for that (we use the same libcvc4
library for both binaries).

[0] https://github.com/CVC4/CVC4/blob/c2111c86973b8a80e20a3fdf3cbd0b2ff0dc7010/src/smt/command.cpp#L756
[1] https://en.cppreference.com/w/cpp/container
[2] https://github.com/CVC4/CVC4/blob/c2111c86973b8a80e20a3fdf3cbd0b2ff0dc7010/src/smt/dump.h#L117