refactor(stm32mp1): remove unused refcount helper functions

Remove stm32mp_incr_shrefcnt(), stm32mp_decr_shrefcnt(),
stm32mp_incr_refcnt() and stm32mp_decr_refcnt() that are unused.
The file is then just removed.

Change-Id: I09ee23c02317df5d8f71cbc355d3ed4a67ce2749
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>