Remove invalid import of `exists` from HDF5

[schillic]

The import of HDF5.exists fails and results in a warning.

WARNING: could not import HDF5.exists into MAT

• HDF5 dropped the function exists pre-v0.16 (https://github.com/JuliaIO/HDF5.jl/pull/694).
• The only supported version of HDF5 is v0.16 since #172.

Hence the import will never work with the latest version of MAT and can be safely removed without breaking anything.

[musm]

Thank you!