[lldb] Remove an outdated comment. NFC.

This comment became outdated in 053eb356
(but was moved along); that commit moved the code and the comment
to a separate function, with a separate local variable
`num_of_bytes_read`. On error, the possibly garbage value is never
copied back to the caller's reference, thus the comment is no longer
relevant (and slightly confusing as is).

Differential Revision: https://reviews.llvm.org/D128226