chore: remove unused `enum FrontendTemplate` #1016

Job	Run time
build (stellar-cli, stellar, ubuntu-20.04, x86_64-unknown-linux-gnu)	14m 43s
build (stellar-cli, stellar, ubuntu-20.04, aarch64-unknown-linux-gnu)	16m 50s
build (stellar-cli, stellar, macos-14, x86_64-apple-darwin)	9m 56s
build (stellar-cli, stellar, macos-12, aarch64-apple-darwin)	13m 32s
build (stellar-cli, stellar, windows-latest, x86_64-pc-windows-msvc, .exe)	21m 45s
build (soroban-cli, soroban, ubuntu-20.04, x86_64-unknown-linux-gnu)	14m 26s
build (soroban-cli, soroban, ubuntu-20.04, aarch64-unknown-linux-gnu)	16m 9s
build (soroban-cli, soroban, macos-14, x86_64-apple-darwin)	10m 8s
build (soroban-cli, soroban, macos-12, aarch64-apple-darwin)	16m 27s
build (soroban-cli, soroban, windows-latest, x86_64-pc-windows-msvc, .exe)	21m 37s
	2h 35m 33s

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