chore: remove no-op 1.15 version checks in templates

[mboersma]

Reason for Change:

Since AKS Engine already supports only Kubernetes v1.15.x or later, these template conditionals have no effect.

Issue Fixed:

Refs #3751

Credit Where Due:

Does this change contain code from or inspired by another project?

• No
• Yes

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

Thanks to @jadarsie for noticing the off-by-one error in #3751, which implies this cleanup.

[codecov]

Codecov Report

Merging #3851 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3851   +/-   ##
=======================================
  Coverage   72.83%   72.83%           
=======================================
  Files         149      149           
  Lines       23171    23171           
=======================================
  Hits        16876    16876           
  Misses       5178     5178           
  Partials     1117     1117           

Impacted Files	Coverage Δ
pkg/engine/templates_generated.go	48.65% <ø> (ø)

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[CecileRobertMichon]

/lgtm

[jackfrancis]

/lgtm

[acs-bot]

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