clean up go mod for go-tfe

Signed-off-by: Paul Thrasher <pthrasher@hashicorp.com>

backend/remote/testdata/plan-cost-estimation/cost-estimate.log

@@ -0,0 +1,5 @@
+Cost estimation:
+
+Waiting for cost estimation to complete...
+Resources: 1 of 1 estimated
+           $25.488/mo +$25.488

go.mod

@@ -39,6 +39,7 @@ require (
 	github.com/golang/mock v1.3.1
 	github.com/golang/protobuf v1.3.2
 	github.com/golang/snappy v0.0.0-20180518054509-2e65f85255db // indirect
+	github.com/google/btree v1.0.0 // indirect
 	github.com/google/go-cmp v0.3.0
 	github.com/google/uuid v1.1.1
 	github.com/gophercloud/gophercloud v0.0.0-20190208042652-bc37892e1968
@@ -132,5 +133,3 @@ require (
 	gopkg.in/ini.v1 v1.42.0 // indirect
 	gopkg.in/yaml.v2 v2.2.2
 )
-
-go 1.13

go.sum

@@ -198,10 +198,6 @@ github.com/hashicorp/go-slug v0.3.0 h1:L0c+AvH/J64iMNF4VqRaRku2DMTEuHioPVS7kMjWI
 github.com/hashicorp/go-slug v0.3.0/go.mod h1:I5tq5Lv0E2xcNXNkmx7BSfzi1PsJ2cNjs3cC3LwyhK8=
 github.com/hashicorp/go-sockaddr v0.0.0-20180320115054-6d291a969b86 h1:7YOlAIO2YWnJZkQp7B5eFykaIY7C9JndqAFQyVV5BhM=
 github.com/hashicorp/go-sockaddr v0.0.0-20180320115054-6d291a969b86/go.mod h1:7Xibr9yA9JjQq1JpNB2Vw7kxv8xerXegt+ozgdvDeDU=
-github.com/hashicorp/go-tfe v0.3.21 h1:JVB+DqX4zevWmaVHGsJfElyScf1ZjbF6EUDLPzU96OE=
-github.com/hashicorp/go-tfe v0.3.21/go.mod h1:SuPHR+OcxvzBZNye7nGPfwZTEyd3rWPfLVbCgyZPezM=
-github.com/hashicorp/go-tfe v0.3.22 h1:Wodv4Y3kiC6bTPHQzhrn026V3HLhfwhc1rVlLY8suKs=
-github.com/hashicorp/go-tfe v0.3.22/go.mod h1:SuPHR+OcxvzBZNye7nGPfwZTEyd3rWPfLVbCgyZPezM=
 github.com/hashicorp/go-tfe v0.3.23 h1:kd9hlFQvGubNF/CpF7T5AP/xU8uLUq8ANbI5xRDVSms=
 github.com/hashicorp/go-tfe v0.3.23/go.mod h1:SuPHR+OcxvzBZNye7nGPfwZTEyd3rWPfLVbCgyZPezM=
 github.com/hashicorp/go-uuid v1.0.0/go.mod h1:6SBZvOh/SIDV7/2o3Jml5SYk/TvGqwFJ/bN7x4byOro=