buffer: remove unreachable code

[Trott]

46f202 introduced changes that cause a previously reachable error
condition to be unreachable. Remove the code.

The error condition checks that byteLength() doesn't return -1. But
that will only happen if byteLength() receives an encoding that is
falsy. However the call to byteLength() is in an else block that is
only reachable if encoding is a string with a length greater than 0.
So byteLength() will never return -1 in this situation.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

buffer

[benjamingr]

Can I see a performance benchmark?

Unreachable code might sometimes have surprising effects on the performance :)

[mscdex]

I think #12439 should be considered instead, which fixes the unintentional backwards incompatibility introduced in #12361, making this particular piece of code reachable again.

[jasnell]

I also prefer #12439 instead.

[jasnell]

#12439 has landed. Closing this. Can reopen if necessary.