configs: remove CONFIG_ARM64_ERRATUM_1188873
The Kconfig symbol ARM64_ERRATUM_1188873 was dropped in v5.2-rc2, see
upstream commit a5325089bd05 ("arm64: Handle erratum 1418040 as a
superset of erratum 1188873"). So drop it from the configuration generation
system too.
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>