Revert "Add btrfs file item tracepoints"

This reverts commit b42b2955e13153b7283f20613f15fe98e6427baf.

It introduces the following warnings:

>>> depmod: WARNING: /lib/modules/4.18.0-rc1+/extra/probes/lttng-probe-btrfs.ko
>>> needs unknown symbol btrfs_get_token_32
>>> depmod: WARNING: /lib/modules/4.18.0-rc1+/extra/probes/lttng-probe-btrfs.ko
>>> needs unknown symbol btrfs_get_token_8
>>> depmod: WARNING: /lib/modules/4.18.0-rc1+/extra/probes/lttng-probe-btrfs.ko
>>> needs unknown symbol btrfs_get_token_16
>>> depmod: WARNING: /lib/modules/4.18.0-rc1+/extra/probes/lttng-probe-btrfs.ko
>>> needs unknown symbol btrfs_get_token_64
>>> make[1]: Leaving directory `/home/efficios/git/linux-percpu-dev'

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>