fix: remove includes = ["."] (#1423)

Using `includes = ["."]` in a top-level BUILD file causes too many odd
behaviors and breakages. For example, it makes `bazel build ...` fail.

The goal was to allow users to include our headers using angle-bracket
includes, which this does. However, there may be a better way to do
this. Until we figure that out, we should remove the odd line.

Author: devjgm

BUILD

@@ -22,8 +22,6 @@ exports_files([
 
 cc_library(
     name = "spanner_client",
-    includes = ["."],
-    tags = ["manual"],
     deps = [
         "//google/cloud/spanner:spanner_client",
     ],