runtime/pprof: remove round-to-file-start adjustment

This causes a problem in the test sometimes. With a mapping like:

00400000-00411000 r--p 00000000 fe:01 4459044                            /tmp/go-build1710804385/b001/pprof.test
00411000-00645000 r-xp 00011000 fe:01 4459044                            /tmp/go-build1710804385/b001/pprof.test

The removed code would make the first mapping 0x400000-0x645000. Tests
then grab the first few addresses to use as PCs, thinking they are in
an executable range. But those addresses are really not in an
executable range, causing the tests to fail.

Change-Id: I5a69d0259d1fd70ff9745df1cbad4d54c5898e7b
Reviewed-on: https://go-review.googlesource.com/c/go/+/424295


TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Keith Randall <khr@golang.org>