feat(transport): remove google-c2p dependence to DirectPath (#1361)

Remove google-c2p dependency to the entire google-api-go-client. Instead, services using DirectPath should add the dependency by itself, like googleapis/google-cloud-go#5090.

Issue: #1283

go.sum

@@ -47,34 +47,26 @@ cloud.google.com/go/storage v1.10.0/go.mod h1:FLPqc6j+Ki4BU591ie1oL6qBQGu2Bl/tZ9
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@@ -147,7 +139,6 @@ github.com/google/pprof v0.0.0-20210601050228-01bbb1931b22/go.mod h1:kpwsk12EmLe
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@@ -162,22 +153,17 @@ github.com/ianlancetaylor/demangle v0.0.0-20200824232613-28f6c0f3b639/go.mod h1:
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@@ -556,12 +542,10 @@ google.golang.org/protobuf v1.26.0/go.mod h1:9q0QmTI4eRPtz6boOQmLYwt+qCgq0jsYwAQ
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