Fix duplicate dot in enum name (when "package" is specified)

[sampajano]

Summary

• Fix a bug introduced in Fix Enum with module in generated TS interface. #1278 where an extra dot is inserted when "package" is specified in the enum being imported.

Changes

• This fix makes StripPrefixString remove the leading dot when stripping out a prefix -- and hence will work in both cases (with and without package).
• Also removed a few now redundant logic to remove leading dots.

Tested

• With both example in Missing dot after enum's namespace in typescript file #1271 and Duplicate dot after enum's namespace in typescript file when using dot in package name #1284

Fixes #1284

[stanley-cheung]

LGTM. Thanks!