simpleperf: don't looking for _text symbol in /proc/kallsyms.

simpleperf looks for _text symbol in /proc/kallsyms to get
kernel address space information. But that info is no longer
needed. As some kernels don't have _text symbol, we should
remove the corresponding code.

Bug: 27814103
Change-Id: I4eb9869b58c51846c99b1204f304cd107a0eb354
3 files changed