Documentation: Purges two outdated known issues.

Unix build instruction has two outdated notes under Known Issues.

The first one is fixed by:
https://bugzilla.xamarin.com/show_bug.cgi?id=29679. (/cc @akoeplinger)

The last one was fixed by a series of organization-wide updates to
bash files by @dougbu via dotnet/aspnetcore#695.

Author: Peter Jas

Documentation/building/unix-instructions.md

@@ -72,7 +72,3 @@ PDBs aren't generated by Roslyn on Unix. https://github.com/dotnet/roslyn/issues
 Test runs are currently disabled when building on Unix. https://github.com/dotnet/corefx/issues/1776
 
 System.Diagnostics.Debug.Tests does not build on Unix. https://github.com/dotnet/corefx/issues/1609
-
-Mono fails when trying to get custom attributes on CoreFx assemblies. https://bugzilla.xamarin.com/show_bug.cgi?id=29679
-
-Some third party dependencies may incorrectly assume that `bash` is always installed in `/bin/`. This can manifest itself in errors like `corefx/packages/dnx-mono.1.0.0-beta5-11760/bin/dnu: not found`. The simplest way to get around this until fixed upstream is simply making the assumption correct: `sudo ln -s /usr/local/bin/bash /bin/bash`.