phlex.gemspec: Drop exe/ mentions #1529

Job	Run time
rubocop	11s
tests (ubuntu-latest, 2.7)	21s
tests (ubuntu-latest, 3.0)	9s
tests (ubuntu-latest, 3.1)	9s
tests (ubuntu-latest, 3.2)	7s
tests (ubuntu-latest, head)	7s
tests (ubuntu-latest, truffleruby-22.2)	36s
tests (ubuntu-latest, truffleruby-22.3)	34s
tests (ubuntu-latest, truffleruby-head)	30s
tests (macos-latest, 2.7)	1m 5s
tests (macos-latest, 3.0)	41s
tests (macos-latest, 3.1)	44s
tests (macos-latest, 3.2)	20s
tests (macos-latest, head)	22s
tests (macos-latest, truffleruby-22.2)	1m 4s
tests (macos-latest, truffleruby-22.3)	1m 21s
tests (macos-latest, truffleruby-head)	46s
	9m 7s

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