Remove duplicate AWS Lambda and Route53 dashboards

The standalone AWS Lambda and Route53 dashboards are fully superseded by
the comprehensive aws/ collection, which is managed by the
cloudwatch-dashboards repository. A graph-set diff confirms the aws/
versions are strict supersets at the metric level (no graphs are lost):

- dashboards/lambda/  -> aws/compute_lambda.json     (AWS/Lambda)
    aws/ adds Dead Letter Errors, Destination Delivery Failures, Iterator Age
- dashboards/route53/ -> aws/networking_route53.json (AWS/Route53)
    aws/ adds Endpoint Latency (connection/SSL/TTFB); renamed graphs only

Drop the matching README entries.

Firehose is intentionally NOT removed: a graph-set diff shows the
standalone firehose/ dashboard has 21 metrics (HTTP endpoint delivery,
KMS key errors, failed validation, PutRecord latency/requests, rate
limits, throttled records) that aws/messaging_firehose.json does not
cover, so deleting it would be a coverage regression.

The AWS CloudWatch Metric Streams dashboard (dashboards/cloudwatch/) is
also kept; it monitors the metric streams ingestion pipeline and has no
equivalent in the aws/ collection.

Author: serenachou

README.md

@@ -11,8 +11,6 @@ Automated Dashboards can be found in the `dashboards/` sub directory. Each direc
 - [AWS](/dashboards/aws/) (via AWS CloudWatch)
 - [AWS CloudWatch Metric Streams](/dashboards/cloudwatch/)
 - [AWS Firehose](/dashboards/firehose/)
-- [AWS Lambda](/dashboards/lambda/)
-- [AWS Route53](/dashboards/route53/)
 - [Ecto](/dashboards/ecto/)
 - [Erlang](/dashboards/erlang/)
 - [Heroku](/dashboards/heroku/)