Remove unnecessary static assertion

[WhyNotHugo]

This assertion is not required. The docs for std::mem::transmute read:

Both types must have the same size. Compilation will fail if this is
not guaranteed.

I have confirmed this to be the case by trying to transmute two different variables of different length:

error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
  --> src/sys/socket/addr.rs:44:14
   |
44 |     unsafe { mem::transmute("asdfasd") }
   |              ^^^^^^^^^^^^^^
   |
   = note: source type: `&str` (128 bits)
   = note: target type: `in_addr` (32 bits)

Given that this is the only usage of the static_assertions crates, this allows dropping an unnecessary dependency.

[stevenengler]

I'm pretty sure this transmute is actually unsafe. I've opened #2053.

[asomers]

Superseded by #2061