chromium: remove redundant `run_mksnapshot_default` build target

This build target, mksnapshot, was added all the way back in 2014 in
8d54dc6d to add support for grsecurity.

A few years later in 2017, grsecurity made their free testing patches
private, forcing nixpkgs to drop the grsecurity-flavored linux
kernel in 32b8512e.

Roughtly another year later, 1b146a8c
removed paxutils from the stdenv and the remaining paxmark invocations
in various build expressions. The explicit mksnapshot build target,
however, remained, for no reason.

In 2023, 0a05fbb9 renamed the target
from mksnapshot to run_mksnapshot_default for cross-compilation reasons.

Note that removing it does not improve compile times in any meaningful
way. This is because mksnapshot is implicitly pulled in by the main
build target, chrome. It does, however, make the ninja task count
more accurate and build dependency errors that would previously only
happen in chrome but not mksnapshot, no longer require to first build
mksnapshot and instead fail immediately.