Fix bug that threw an error while searching for linux headers through a symlink

[kpattaswamy]

Summary: This PR removes a condition which checks to see if a host path exists in the filesystem of a process when trying to resolve symlinks.

The function fs::Exists() returned false for a broken symlink when the expectation was to return true because the path existed although it was a broken symlink. This function would later on fix the broken symlink path by reading its target host and converting the broken path into the actual path. This makes the code removal safe as the next check: is_symlink will return an error code if there is no file.

Type of change: /kind bug

Test Plan: Existing tests pass and deployed the PEM with the updated fix and saw that the Linux header files were found.

[etep]

We should update the error message when is_symlink returns an error. Something like:

return error::NotFound(absl::Substitute("Did not find host headers at path: $0, $1.", p.string(), ec.message()));

[etep]

Can we try a PR description along the lines of:

Method fs::Exists returned false for a broken symlink, but its expected behavior was to return true, i.e. because something existed (even if that something was a broken symlink). This function later fixes the broken symlink by reading its target and converting that target into a host path.

Removing the fs::Exists check solves the bug where the function early exits before resolving a symlink (see above explanation) and is safe because the next check, is_symlink will return an error code if there is no file.

[etep]

Also, we can update the test plan, something like:

deployed pem with bug fix and observed correct behavior.

[etep]

My own comment:

The condition, removed in this PR, was fully redundant to the next check is_symlink. Not only was it redundant, but it was also wrong in the case of a broken symlink.