parsing: Remove implicit frames created for nested models

[azeey]

This removes additional implicit model frames created for nested models that were missed in #15099.

/cc @EricCousineau-TRI

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This change is 

[EricCousineau-TRI]

@drake-jenkins-bot retest this please

[SeanCurtis-TRI]

+@EricCousineau-TRI for feature review, please.

Reviewable status: LGTM missing from assignee EricCousineau-TRI(platform), needs at least two assigned reviewers (waiting on @EricCousineau-TRI)

[EricCousineau-TRI]

Sorry for delay, and looks good! Though needs downstream testing.

As this is a small change, I'm fine with serving as platform as well if there are no downstream issues.

Reviewed 2 of 2 files at r1.
Reviewable status: 1 unresolved discussion, LGTM missing from assignee EricCousineau-TRI(platform), needs at least two assigned reviewers (waiting on @EricCousineau-TRI)

a discussion (no related file):
Working: Testing on Anzu CI.

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[EricCousineau-TRI]

+(status: single reviewer ok)
x 2 given that we have not broken Anzu CI - thanks!!!

Reviewable status: commits need curation (https://drake.mit.edu/reviewable.html#curated-commits) (waiting on @azeey)

a discussion (no related file):

Previously, EricCousineau-TRI (Eric Cousineau) wrote…

Working: Testing on Anzu CI.

Done. Anzu CI just passed; PR 7180, job 1. Sweet!

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[EricCousineau-TRI]

CI failure seems like a flake: https://drake-jenkins.csail.mit.edu/job/linux-bionic-gcc-bazel-experimental-release/9964/consoleFull

[5:00:38 PM]  INFO: From Testing //manipulation/util:py/show_model_test:
...
[5:00:38 PM]  FileNotFoundError: [Errno 2] No such file or directory: '/home/ubuntu/.cache/matplotlib/fontList.json'
...
[5:00:38 PM]  OSError: [Errno 30] Read-only file system: '/home/ubuntu/.cache/matplotlib/fontList.json'

Retesting:
@drake-jenkins-bot linux-bionic-gcc-bazel-experimental-release please

[EricCousineau-TRI]

Yup, just a flake: https://drake-jenkins.csail.mit.edu/job/linux-bionic-gcc-bazel-experimental-release/9965/console