fusefs: Remove an unused pbuf zone

The zone has been dead ever since commit
b9e20197551d ("fusefs: rewrite vop_getpages and vop_putpages")

No functional change intended.

Reviewed by: asomers
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D40143

(cherry picked from commit e3f7081b1c54fae55cb443a39c2ad44ac3ed4a75)