OK, given that info, try this:
-- select the top level folder
set base_folder to (choose folder)
tell application "Finder"
-- get a list of the folders in here
set folder_list to every folder of folder base_folder
-- iterate through them
repeat with each_folder in folder_list
-- get the current folder name
set fn to name of each_folder
-- does it contain the string we're looking for?
if (fn contains "(") and (fn contains "AB)") then
-- find the start
set i to offset of "(" in fn
-- and the end
set j to 3 + (offset of "AB)" in fn)
-- construct the new folder name without the (xxx AB) part
set new_name to characters 1 through (i - 1) of fn
if j < (count characters of fn) then
set new_name to new_name & (characters j through -1 of fn)
end if
-- and rename the folder
set name of each_folder to (new_name as text)
end if
end repeat
end tell
The comments should be self-explanatory, but let me know if you need help understanding the flow.