fastbootd.te: Delete allowxperm statement

system/sepolicy commit 4c8eaba7, reviewed in
https://android-review.googlesource.com/c/platform/system/sepolicy/+/793958
started enforcing explicit ioctl permission checks for all block device
files. As part of that commit, the following lines were added to
domain.te:

  # If a domain has access to perform an ioctl on a block device, allow these
  # very common, benign ioctls
  allowxperm domain dev_type:blk_file ioctl { BLKGETSIZE64 BLKSSZGET };

In essence, if a domain is granted ioctl access to any device in
policy (for example, via adding "ioctl" to the allow rule, or by using
the macro "r_file_perms" which includes the ioctl permission), then the
two ioctls BLKGETSIZE64 and BLKSSZGET will be automatically allowed. As
such, it is redundent for a domain to explicitly request these two
ioctls.

Delete the now redundant allowxperm rule.

Test: policy compiles
Change-Id: I1964ed93a7c7601393cc9e2416f3640ea22db51b