Fix(GraphQL): Add extra checks for deleting UpdateTypeInput

[vmrajas]

Motivation:
A user had reported panic on schema update. The reason was that cleanupInput function cleared any input types which had name of the form, UpdateTInput and contained exactly one field. This PR adds extra conditions to ensure that the input type is removed only when its generated by dgraph and contains a field by name, filter.

Testing:
Added unit test in graphql/schema/testdata/schemagen

Fixes GRAPHQL-1102

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This change is 

[abhimanyusinghgaur]

Reviewed 3 of 3 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @pawanrawal and @vmrajas)

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graphql/schema/gqlschema.go, line 1073 at r1 (raw file):

	if strings.HasPrefix(def.Name, "Update") && len(def.Fields) == 1 {
		if def.Fields[0].Name == "filter" {

we can make the condition exactly match what we generate, like this:

if strings.HasPrefix(def.Name, "Update") && strings.HasSuffix(def.Name, "Input") && len(def.Fields) == 1 {
  typeDef := sch.Types[def.Name[6:len(def.Name)-5]]
  if typeDef != nil && typeDef.Directives.ForName(remoteDir) == nil && (typeDef.Kind == ast.Object || typeDef.Kind == ast.Interface) {
    // this ensures that it was us who generated the `UpdateTInput`
    // and allows users to still be able to define a type `UpdateT1Input` with a field named `filter` in that input type and not get cleaned up.
  }
}