Remove the compatibility variable name "__bp_install_string"

Author: akinomyoga

bash-preexec.sh

@@ -62,10 +62,6 @@ __bp_inside_preexec=0
 # Initial PROMPT_COMMAND string that is removed from PROMPT_COMMAND post __bp_install
 bash_preexec_install_string=$'__bp_trap_string="$(trap -p DEBUG)"\ntrap - DEBUG\n__bp_install'
 
-# WARNING: This variable is no longer used and should not be relied upon.
-# Use ${bash_preexec_install_string} instead.
-__bp_install_string="${bash_preexec_install_string}"
-
 # The command string that is registered to the DEBUG trap.
 bash_preexec_trapdebug_string='__bp_preexec_invoke_exec "$_"'