bt(4): Remove support for ISA adapters.

bt(4): Remove support for ISA adapters.

The driver also supports PCI cards and we do want to keep this support,
because it is offered by current VirtualBox, for example.

• [DBH] share/man/man4/bt.4
• [DBH] share/man/man4/isa.4
• [DBH] sys/conf/files
• [DBH] sys/config/GENERIC
• [DBH] sys/config/LINT
• [DBH] sys/config/LINT64
• [DBH] sys/config/X86_64_GENERIC
• [DBH] sys/dev/disk/buslogic/bt.c
• [DBH] sys/dev/disk/buslogic/bt_isa.c
• [DBH] sys/dev/disk/buslogic/bt_pci.c
• [DBH] sys/dev/disk/buslogic/btreg.h
• [DBH] sys/platform/pc32/i386/userconfig.c