stand: Remove dead store to bi_kernelname

We set this value twice: once to 0 and once to the VA that has the name
of the kernel. The first store is redundant. In addition, these two
stores of 0 are also redundant. Since we never set them, they will
always be zero, even if we're called multiple times. This saves 21
bytes on BIOS loader.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D36541