stv0991: remove specific CONFIG_STV0991 configs

Remove the following STV0991 specific configs:
- CONFIG_STV0991 (never used, only defined in CONFIG_SYS_EXTRA_OPTIONS)
- CONFIG_STV0991_HZ (replaced by generic CONFIG_SYS_HZ)
- CONFIG_STV0991_HZ_CLOCK (replaced by generic CONFIG_SYS_HZ_CLOCK)

This patch allows to reduce the file config_whitelist.txt.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>