Arch Linux

The latest alembic assumes it's built against external `imath` library instead of one bundled in `openexr`
https://github.com/alembic/alembic/commit/69ede84b6c4906031f566c1e72fc6bb56c638354
Since we don't separate `imath` from `openexr` we have to remove it from `AlembicConfig.cmake`
Otherwise cmake projects (e.g: AUR/meshroom) fails with:
```
CMake Error at /usr/share/cmake-3.20/Modules/CMakeFindDependencyMacro.cmake:47 (find_package):
By not providing "FindIMath.cmake" in CMAKE_MODULE_PATH this project has
asked CMake to find a package configuration file provided by "IMath", but
CMake did not find one.

Could not find a package configuration file provided by "IMath" with any of
the following names:

IMathConfig.cmake
imath-config.cmake

Add the installation prefix of "IMath" to CMAKE_PREFIX_PATH or set
"IMath_DIR" to a directory containing one of the above files. If "IMath"
provides a separate development package or SDK, be sure it has been
installed.
Call Stack (most recent call first):
/usr/lib64/cmake/Alembic/AlembicConfig.cmake:39 (find_dependency)
CMakeLists.txt:26 (find_package)
```
On the other hand, we could follow `openexr` packaging scheme and package the imath separately.
https://github.com/AcademySoftwareFoundation/openexr#openexr-and-imath-version-3