kernel: Remove some old ISA only drivers.

kernel: Remove some old ISA only drivers.

This commits removes the following old ISA specific drivers, most of which
have been removed in FreeBSD, too:

aha(4)    - Adaptec 154xA, 154xB, 154xC, 154xCF and 154xCP SCSI cards
asc(4)    - GI1904-based hand scanner
ctx       - ImageNation CORTEX-I Frame Grabber
dgb(4)    - DigiBoard serial cards (digi(4) driver remains)
el(4)     - 3Com Etherlink 3C501 NIC
gpib      - National Instruments AT-GPIB and AT-GPIB/TNT boards
gsc(4)    - Genius GS-4500 hand scanner
ie(4)     - Intel i82586 based 8 and 16 bit ISA ethernet cards
labpc(4)  - National Instruments LABPC and LABPC+ driver
le(4)     - DEC EtherWORKS II/III ethernet cards
mse(4)    - Bus and InPort mouse driver
rc(4)     - RISCom/8 multiport serial cards
rdp(4)    - RealTek RTL8002 based pocket ethernet adapters
spigot    - Creative Labs Spigot video acquisition board
tw(4)     - TW-523 X-10 interface
wl(4)     - T1 speed ISA/radio LAN card
wt(4)     - Archive/Wangtek cartridge tape driver

Along with those, a number of associated programs are removed, too:

sasc(1) sgsc(1) wlconfig(8) xten(1) xtend(8)