components: Remove passing of CMAKE_BUILD_TYPE to ExternalProject

There are two reasons to *not* pass `CMAKE_BUILD_TYPE` to a sub build of
`ExternalProject`:

* Passing of `CMAKE_BUILD_TYPE` from a main CMake project build to an
  `ExternalProject` build does not work when `CMAKE_BUILD_TYPE` is not
  explicitly set by the user. When `CMAKE_BUILD_TYPE` is empty, CMake
  will select the main project's and external project's respective
  default build types which may be different. For example, the type is
  `RelWithDebInfo` for the Open IoT SDK and `MinSizeRel` for TF-M.
* The user should have the freedom to build main project and external
  projects with different types, such as enabling debug symbols in the
  main application without unnecessarily doing the same for TF-M.

If the user wants to change the external project's build type, they can
add `CMAKE_BUILD_TYPE` to `TFM_CMAKE_ARGS` or `ML_CMAKE_ARGS` as needed.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>