internal: delete unused experiments

Change-Id: I06e75bc367051412ace7711b42948e8f801a5524
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/297211
Trust: Julie Qiu <[email protected]>
Run-TryBot: Julie Qiu <[email protected]>
TryBot-Result: kokoro <[email protected]>
Reviewed-by: Jonathan Amsterdam <[email protected]>

internal/experiment.go

@@ -7,25 +7,21 @@ package internal
 
 const (
 	ExperimentCommandTOC            = "command-toc"
-	ExperimentDirectoryTree         = "directory-tree"
 	ExperimentInsertSymbols         = "insert-symbols"
 	ExperimentInsertSymbolHistory   = "insert-symbol-history"
 	ExperimentInteractivePlayground = "interactive-playground"
 	ExperimentNotAtLatest           = "not-at-latest"
-	ExperimentRedirectedFromBanner  = "redirected-from-banner"
 	ExperimentRetractions           = "retractions"
 )
 
 // Experiments represents all of the active experiments in the codebase and
 // a description of each experiment.
 var Experiments = map[string]string{
 	ExperimentCommandTOC:            "Enable the table of contents for command documention pages.",
-	ExperimentDirectoryTree:         "Enable the directory tree layout on the unit page.",
 	ExperimentInsertSymbols:         "Insert data into symbols, package_symbols, and documentation_symbols.",
 	ExperimentInsertSymbolHistory:   "Insert symbol history data into the symbol_history table.",
 	ExperimentInteractivePlayground: "Enable interactive example playground on the unit page.",
 	ExperimentNotAtLatest:           "Enable the display of a 'not at latest' badge.",
-	ExperimentRedirectedFromBanner:  "Display banner with path that request was redirected from.",
 	ExperimentRetractions:           "Retrieve and display retraction and deprecation information.",
 }