cpu/esp32: fix esp_wifi_enterprise compilation problem

With the change from using archive files to using object files in PR RIOT-OS#14754, the module `esp_wifi_enterprise` was no longer compilable. The reason for this was that the file `bignum.c` was present in two different vendor sub-directories and created two identical object files, which then led to a symbol conflict when linking. This commit removes one of these identical files. The one that is used in all `esp_wifi` variants is kept, the one that is only used in `esp_wifi_enterprise` is dropped.