doc: Remove outdated CentOS comment

No longer applicable after CONFIG_SHELL must be explicitly set to dash
after commit fafc55a489.

ci/test/06_script_b.sh

@@ -52,9 +52,6 @@ fi
 
 if [ -z "$NO_DEPENDS" ]; then
   if [[ $CI_IMAGE_NAME_TAG == *centos* ]]; then
-    # CentOS has problems building the depends if the config shell is not explicitly set
-    # (i.e. for libevent a Makefile with an empty SHELL variable is generated, leading to
-    #  an error as the first command is executed)
     SHELL_OPTS="LC_ALL=en_US.UTF-8 CONFIG_SHELL=/bin/dash"
   else
     SHELL_OPTS="CONFIG_SHELL="