Remove redundant "default" descriptions from help output

[flimzy]

Diff of before/after output:

--- before      2020-10-15 08:28:33.285100444 +0200
+++ after       2020-10-15 08:30:53.983319078 +0200
@@ -30,7 +30,7 @@
       --charts strings                 Specific charts to test. Disables changed charts detection and
                                        version increment checking. May be specified multiple times
                                        or separate values with commas
-      --check-version-increment        Activates a check for chart version increments (default: true) (default true)
+      --check-version-increment        Activates a check for chart version increments (default true)
       --config string                  Config file
       --debug                          Print CLI calls of external tools to stdout (caution: setting this may
                                        expose sensitive data when helm-repo-extra-args contains passwords)
@@ -49,7 +49,7 @@
       --remote string                  The name of the Git remote used to identify changed charts (default "origin")
       --since string                   The Git reference used to identify changed charts (default "HEAD")
       --target-branch string           The name of the target branch used to identify changed charts (default "master")
-      --validate-chart-schema          Enable schema validation of 'Chart.yaml' using Yamale (default: true) (default true)
-      --validate-maintainers           Enable validation of maintainer account names in chart.yml (default: true).
+      --validate-chart-schema          Enable schema validation of 'Chart.yaml' using Yamale (default true)
+      --validate-maintainers           Enable validation of maintainer account names in chart.yml.
                                        Works for GitHub, GitLab, and Bitbucket (default true)
-      --validate-yaml                  Enable linting of 'Chart.yaml' and values files (default: true) (default true)
+      --validate-yaml                  Enable linting of 'Chart.yaml' and values files (default true)

What this PR does / why we need it:

The help output (and .md docs created from it) contains redudant text.

[unguiculus]

Thanks