This is not a question of textual criticism, nor is there any reason to reject the authenticity of Mark 7:19. It is entirely a question of interpreting the text. Let us look at the oldest versions:

The Greek original has:

οτι ουκ εισπορευεται αυτου εις την καρδιαν αλλ εις την κοιλιαν και εις τον αφεδρωνα εκπορευεται καθαριζων παντα τα βρωματα

The Pshitta has:

ܕ݁ܠܳܐ ܗ݈ܘܳܐ ܠܠܶܒ݁ܶܗ ܥܳܐܶܠ ܐܶܠܳܐ ܠܟ݂ܰܪܣܶܗ ܘܡܶܫܬ݁ܕ݂ܶܐ ܒ݁ܬ݂ܰܕ݂ܟ݁ܺܝܬ݂ܳܐ ܕ݁ܰܡܕ݂ܰܟ݁ܝܳܐ ܟ݁ܽܠܳܗ ܡܶܐܟ݂ܽܘܠܬ݁ܳܐ ܀

The Old Syriac (Vetus Syrus) has:

ܡܛܠ ܕܠܐ ܗܘܐ ܓܝܪ ܠܠܒܐ ܥܐܠ ܐܠܐ ܠܟܪܣܐ ܘܡܫܬܕܐ ܠܒܪ ܘܡܬܕܟܐ ܟܘܠܗ ܡܐܟܠܐ

And the Vulgata has:

quia non intrat in cor eius, sed in uentrum uadit, et in secessum exit, purgans omnes escas

And compare the KJV:

Because it entereth not into his heart, but into the belly, and goeth out into the draught, purging all meats?

The participle καθαριζων “purifying, purging” can be construed either with αφεδρων “excretive system”, or with the subject of the verb “he said”, that is: Jesus. So the Greek text can mean either that the intestines purify the foods, or else the quotation can end with the word εκπορευεται, followed by the statement that “he” (Jesus) thereby was “purifying all foods”. The Latin has the same ambiguity (purgans can go with secessum, or with the subject of inquit), but the Pshitta allows only the former interpretation (ܕ݁ܰܡܕ݂ܰܟ݁ܝܳܐ is feminine and must therefore go with ܬ݂ܰܕ݂ܟ݁ܺܝܬ݂ܳܐ). The Vetus Syrus, however, allows both interpretations, though the latter (with Jesus as the referent) seems very forced.