Remove dead duplicate function definitions

[billdenney]

These were defined twice; R sources files alphabetically so the later one wins and the earlier copy is dead code:

• .DollarNames.rxEt: DollarNames.R copy is masked by the richer et.R copy (which adds an is.rxEt() guard). Removed the DollarNames.R copy; the NAMESPACE S3method still resolves to et.R's. Kept .DollarNames.rxSolve.
• symengine helpers unknownCsymengine, symengineCEnv, seC, allNames, allCalls, cloneEnv, .exists2: dsl.R copies are masked by the symengine.R copies (which every caller -- rxSymInv.R, err.R, rxOptExpr.R, rbind.R -- already resolves to via the namespace). Removed the dead dsl.R copies; kept the unique dsl.R functions sympyTransit4 and allStrs.